RESEARCH
UPDATE
CHRONOBIOLOGICAL
REGULATION OF ALCOHOL INTAKE
Susanne Hiller-Sturmhöfel,
Ph.D.,
SUSAN HILLER-STURMHOFEL, PH.D., is a science editor of
Alcohol Research & Health. PAUL KULKOSKY, PH.D., is a professor
of psychology in the Department of Psychology, |
Like
other physiological functions, food intake and metabolism (including alcohol
consumption) in humans and animal models may be regulated by circadian rhythm.
For example, many studies of rodents have found that alcohol consumption in
these nocturnal animals peaks during their active dark period. This alcohol
consumption pattern can be influenced, however, by experimental manipulation.
One factor that has been proposed to play a role in regulating circadian alcohol
consumption pattern is the hormone melatonin, which is produced by the pineal
gland. Research also indicates that the effects of lighting conditions on
the alcohol consumption of animal models may be influenced by the differences
among the strains of the laboratory animals used, variations in the type and
administration schedule of the animals alcohol-containing diet, disruptions
of the normal circadian rhythm, concurrent use of other drugs, and properties
of the light.
KEY WORDS: circadian rhythm; biological regulation; light; time of day; AOD
(alcohol or other drug) use pattern; pineal gland; melatonin; cholecystokinin;
animal model; laboratory rat
The
activity patterns and body functions of humans (like those of other animals)
are, at least in part, regulated by general environmental influences, such
as temperature and lighting conditions. In particular, the daily light-dark
cycle (also called the diurnal cycle) has shaped the activity patterns of
most animals over millions of years of evolution. Thus, humans generally are
active during the day-light and rest during darkness (although modern technology,
such as the invention of electrical light, has vastly modified those natural
activity patterns). Conversely, rodents are primarily nocturnal animals. These
general activity patterns influence numerous other behaviors, including food
consumption and metabolism. Accordingly, alcohol consumption patterns in humans
and laboratory animals may be affected by the circadian1 (1 The term circadian rhythm
refers to any pattern that is based on an (approximately) 24-hour cycle. Conversely,
the term diurnal rhythm refers to patterns of activity or behavior
that follow day-night cycles, such as the breakfast-lunch-dinner schedule.
) (i.e., lasting approximately 24 hours) rhythm and the daily light-dark
cycle.
This article first describes briefly how researchers assess the alcohol consumption patterns of laboratory animals across the diurnal cycle. It then reviews the influence of daily lighting conditions on the alcohol consumption of rodents and explores the biological mechanisms that may underlie these influences. Finally, the article presents some of the factors that influence the relationship between lighting conditions and alcohol consumption. This discussion briefly describes the implications of these studies for alcohol consumption patterns in humans, particularly in people whose regular circadian rhythm is frequently disrupted (e.g., shift workers or people traveling across different time zones). Many of the topics discussed in this article generated considerable interest primarily in the 1970s and 1980s but, despite their potential relevance to human alcohol consumption patterns, have not yet been thoroughly pursued. As a result, at least part of the literature reviewed in this article is relatively old, although this does not negate its validity.
To determine the influence of
lighting conditions and diurnal drinking patterns of laboratory animals, researchers
must regularly monitor the animals water (and/or alcohol) consumption.
Some studies in this area have focused on the animals overall fluid
consumption per day under various lighting conditions (e.g., normal light-dark
cycles, continuous darkness, or continuous light). For these experiments,
the animals have constant access to drinking bottles filled with a specific
amount of water and/or an alcohol solution, and their fluid consumption is
measured one or more times per day.
When
the goal of the experiment is to determine in detail the animals consumption
patterns throughout the day, however, such an approach is not adequate, particularly
if such measurements are to be conducted over several days. Moreover, several
bottle changes per day would require waking up the animals periodically, which
by itself could influence their drinking behavior and thereby confound the
results. Therefore, scientists have developed various types of apparatus that
continuously measure fluid consumption and record that information either
on a paper printout or, more recently, on a computer. For example, Eriksson
(1972) designed a device that measures the fluid level in a laboratory drinking
bottle using an electrical probe that is immersed in the bottle and which
records specific changes in signal voltage as the fluid level in the bottle
decreases. Other investigators have used specially designed cages and drinking
bottles that are connected to an electrical circuit and which generate a signal
each time the animal licks at the spout of the bottle to receive fluid (e.g.,
Freund 1970; Dole et al. 1983). Especially with the use of computers to automatically
collect, store, and analyze the data, the most current devices can provide
investigators with an accurate picture of the times and amounts of fluid consumed
by each animal under various environmental conditions. As a result, techniques
exist today that allow simultaneous microstructural analysis of the animals
alcohol, fluid, and food intake (Boyle et al. 1997; Reidelberger et al. 1996).
These newer techniques are much more precise and accurate than the earlier
procedures previously described and often reveal differences as well as similarities
in how and why rats consume alcohol in comparison with humans (Dole et al.
1985; Sinclair 1980).
Some researchers have noticed that the general pattern of food and fluid consumption in rodents resembles a 24-hour sinusoidal curve, with peak levels of consumption around the middle of the dark phase and the lowest (i.e., trough) levels around the middle of the light phase. Based on this observation, investigators can generate a curve that reasonably reflects the animals food and fluid (including alcohol) intake based on just a few measurements conducted at evenly spaced intervals throughout the day (Goldstein and Kakihana 1977). For animals consuming alcohol, this approach can also be used to estimate blood alcohol concentrations (BACs) throughout the day based on a few (e.g., three) measurements taken over a period of 24 hours. Although this approach is not accurate enough for detailed chronobiological studies, it provides a relatively simple technique for studies with a more pharmacological focus (e.g., studies that assess the association of BACs with other physiological processes).
The
Influence of Lighting Conditions on Alcohol Consumption Patterns in Rodents
As
scientists began to use rodents as animal models for human alcohol consumption,
they also studied the animals diurnal alcohol consumption patterns.
These studies frequently were motivated by the researchers need to identify
environmental factors that influence the animals level of alcohol self-administration.
In addition, researchers needed to establish whether the diurnal changes in
BACs and the resulting physiological effects of alcohol in the animals adequately
mirror those in human alcoholics.
The
earliest investigations of diurnal consumption patterns determined that when
the alcohol is provided in the form of solutions containing between 7.5 and
25 percent alcohol in water (either as the only source of fluids or in addition
to drinking water), most of the alcohol consumption (i.e., approximately 65
to 75 percent) occurs during the active dark period (Freund 1970; Hatton and
Vieth 1974). This consumption pattern is similar to that observed for normal
drinking water, as further demonstrated by experiments comparing the drinking
patterns of an alcohol-preferring line of rats (i.e., AA rats) and a non-alcohol-preferring
line (i.e., ANA rats) (Eriksson 1972). In those experiments, all animals were
kept in an environment with 12 hours of light (from 6:00 a.m. to 6:00 p.m.)
and 12 hours of darkness and had unlimited access to food, water, and an alcohol
solution. Under these conditions, animals from both strains showed three peaks
of fluid consumption at around 6-7 p.m., 11 p.m., and 3-4 a.m. The only difference
between the strains was that the AA animals primarily consumed the alcohol
solution, whereas the ANA rats almost exclusively consumed water. This consumption
pattern with three distinct peaks of fluid consumption during the dark phase
also was confirmed in another strain of alcohol-consuming rats, called Sardinian
alcohol-preferring rats (Agabio et al. 1996), and in C57BL mice (Millard and
Dole 1983).
Thus,
alcohol consumption in rats and other rodents normally follows a distinct
circadian pattern that coincides with the animals general activity and
consummatory behavior patterns. Experimental manipulation, however, allows
researchers to induce alcohol consumption that is evenly distributed across
the day. For example, when rats receive alcohol as part of a liquid diet that
is the animals only source of food and fluids, the nocturnal pattern
of alcohol consumption is greatly reduced (i.e., the animals consume large
quantities of the diet both at night and during the day), possibly because
the animals cannot obtain enough food during their normal feeding sessions
to meet their nutritional needs (Freund 1970, Reidelberger et al. 1996). (Most
liquid diets have a very sweet taste; consequently, rodents possibly consume
those liquid diets throughout the 24-hour cycle because of their preference
for drinking sweet solutions.) Accordingly, researchers conducting animal
experiments must adjust the mode of alcohol delivery to the aim of their experiments
(e.g., whether they want to achieve constant BACs throughout the day or whether
they are interested in the consequences of fluctuating BACs).
The
Potential Role of the Pineal Gland in Circadian Patterns of Alcohol Consumption
Because
numerous studies had confirmed the distinct nocturnal alcohol consumption
pattern that normally prevails in rodents, scientists throughout the 1970s
sought to identify the mechanisms underlying this pattern. These investigations
focused on the pineal gland, which is located in the brain, and its primary
hormone product, melatonin. This hormone is secreted into the bloodstream
and distributed throughout the body, where it influences the actions of numerous
other hormones as well as exerts other effects (e.g., appears to decrease
skin pigmentation). The secretion of melatonin into the blood follows a marked
diurnal pattern, with blood levels in humans approximately 10 times greater
at night than during the day. These observations, together with findings that
rats kept in constant darkness have larger pineal glands and exhibit greater
activity of the melatonin-forming enzyme (Geller 1971), suggest that the pineal
gland and melatonin may play a role in determining diurnal drinking patterns.
To
investigate this hypothesis, researchers have primarily used two approaches:
(1) removing the pineal glands of laboratory animals (e.g., rats and hamsters)
to eliminate the influence of the bodys own (i.e., endogenous) melatonin
and/or (2) treating the animals with additional (i.e., exogenous) melatonin
to determine the effects of increased melatonin levels. The results of these
investigations, however, have been rather inconsistent and allow no firm conclusions
about the role of the pineal gland and melatonin in determining diurnal drinking
patterns of rodents. Moreover, most of the studies conducted in this area
are rather old, and the issue has not been investigated further in recent
years. The following paragraphs summarize the research results obtained to
date.
Geller
(1971) treated two rats that showed no preference for alcohol under a normal
light-dark cycle with melatonin using daily injections for 2 weeks. In those
animals, alcohol consumption increased and water consumption decreased over
those 2 weeks. However, because this experiment involved only two rats, its
value is limited (see Sinclair 1972).
Blum
and colleagues (1973) compared the alcohol consumption patterns of rats whose
pineal glands had been removed with those of control animals both in total
darkness and under a normal light-dark cycle. When placed in total darkness,
the control animals showed significantly increased alcohol consumption and
reduced water consumption. The animals whose pineal glands had been removed,
despite showing some increase in alcohol consumption, consistently drank less
alcohol and significantly more water than did the control animals. These findings
suggest that the pineal gland indeed modulates the darkness-induced alcohol
preference in rats. Treatment with exogenous melatonin, however, did not alter
the alcohol or water intake of the animals whose pineal glands had been removed,
possibly because the melatonin levels achieved were not sufficient to affect
alcohol-drinking behavior or because the animals were already consuming relatively
high alcohol levels that could not be increased further.
Reiter
and colleagues (1974) obtained similar results when conducting the corresponding
experiments in hamsters, which generally show a greater preference for alcohol
than do rats. Thus, removal of the pineal gland reduced the animals
darkness-induced alcohol preference (as well as their alcohol preference during
a normal light-dark cycle), although exogenous melatonin administration had
no effect on the hamsters alcohol preference, again possibly because
the animals alcohol preference already was relatively high or because
melatonin is relatively ineffective as a hormone in hamsters.
Burke
and Kramer (1974), however, found just the reverse effects of pineal gland
removal and exogenous melatonin administration. In that study, removal of
the pineal gland of rats did not significantly alter the animals alcohol
preference, although animals without a pineal gland consumed somewhat less
alcohol than did the control animals. Administration of exogenous melatonin
to rats that had an intact pineal gland but exhibited a low alcohol preference,
however, substantially increased the animals alcohol consumption.
Several
factors may influence the outcome of studies evaluating the role of melatonin
and the pineal gland and thus contribute to the discrepancies among the results.
One of those factors may be the choice of animal models (Rudeen and Symmes
1981). Thus, studies conducted in rats found that exogenous melatonin enhanced
alcohol consumption (Geller 1971; Smith et al. 1980), whereas in hamsters
exogenous melatonin had no effect (Reiter et al. 1974) or even reduced alcohol
consumption (Rudeen and Symmes 1981).
Another
factor may be the timing of the melatonin administration. For example, Smith
and colleagues (1980) noted that when they administered melatonin to rats
during the dark phase of the daily cycle, the animals self-administration
of alcohol increased significantly. When the melatonin was administered during
the light phase, however, it had only a weak, statistically not significant
effect on alcohol consumption. The investigators speculated that if melatonin
is administered during the dark phase, when the animals internal melatonin
levels are already high, the additional melatonin may boost overall hormone
levels above the limits needed to achieve an effect. Conversely, if the melatonin
is administered during the light phase, overall hormone levels may still be
too low to promote alcohol consumption. Accordingly, researchers must consider
such issues when designing their experiments.
Burke
and Kramer (1974) speculated on potential mechanisms through which the exogenous
melatonin could influence alcohol consumption. First, melatonin itself or
one of its breakdown products may interact with alcohol or its breakdown product
(i.e., acetaldehyde) to form chemicals that may stimulate alcohol preference.
Second, exogenous melatonin might alter the normal levels of the brain chemical
(i.e., neurotransmitter) serotonin, which may play a role in tolerance, withdrawal,
and intoxication. Serotonin is a precursor for melatonin--that is, through
several metabolic steps, serotonin is converted to melatonin in the pineal
gland. Administration of exogenous melatonin may reduce the bodys own
rate of melatonin formation, thereby resulting in elevated serotonin levels,
which in turn might influence alcohol preference.
This relationship between melatonin levels, serotonin levels, and alcohol preference was further investigated by Geller and Hartman (1977), who treated hamsters with a serotonin precursor called 5-hydroxytryptophan (5-HTP). When the animals received 5-HTP injections for several days, alcohol consumption declined substantially in some of the animals, suggesting that changes in serotonin levels and/or the resultant changes in melatonin levels may influence alcohol consumption patterns in hamsters.
Factors
Influencing the Effects of Lighting Conditions on Alcohol Consumption
More recently, researchers have
focused on the analysis of various factors that might influence diurnal drinking
patterns of laboratory animals. Such factors include strain differences; differences
in the type of alcohol-containing diet or its administration schedule; disruption
of the normal daily light-dark cycle; concurrent administration of other drugs;
the bodys own signaling systems, such as hormones; and the properties
(e.g., wavelength) of the light used.
Strain
Differences
As
mentioned earlier in this article, substantial differences in alcohol consumption
patterns exist among various types of rodents and even among different strains
of the same type of rodent. For example, hamsters generally have a higher
preference for alcohol than do many commonly used strains of rats (Reiter
et al. 1974; Rudeen and Symmes 1981). Similarly, some rat strains have a significantly
higher preference for alcohol than do other strains (Eriksson 1972; Aalto
1986).
Strain
differences also may exist in the specific diurnal consumption patterns. For
example, although initial analyses of the alcohol-preferring AA rats and the
nonalcohol-preferring ANA rats had indicated that both strains had similar
consumption patterns of alcohol and water (Eriksson 1972), detailed analyses
using more sophisticated measuring techniques subsequently demonstrated that
some differences do exist (Aalto 1986). Thus, the AA rats showed three peaks
of alcohol consumption during the dark period that coincided with smaller
peaks in water consumption. Conversely, the ANA rats exhibited two major peaks
in water consumption during the dark phase and rather evenly distributed,
low-level alcohol consumption throughout the dark phase.
Similarly,
a comparison of two rat strains called Fisher rats and spontaneous hypertensive
(SP) rats found that the SP rats demonstrated clear diurnal consumption patterns
for both alcohol and water, regardless of whether the animals were kept on
a normal light-dark cycle or in complete darkness (Pasley et al. 1987). Conversely,
the Fisher rats exhibited a circadian consumption pattern only for water when
they were kept under a light-dark cycle. The animals exhibited no such pattern,
however, for water when they were kept in complete darkness or for alcohol
under any lighting condition (i.e., their alcohol and water consumption under
these conditions remained constant throughout the day).
Although
such differences in alcohol consumption patterns might seem trivial, for some
experiments researchers should be familiar with the consumption patterns of
the specific animal strains they are using, because those patterns also determine
the animals BACs throughout the day. For example, if investigators wish
to determine the animals peak BACs (e.g., to study whether those BACs
are high enough to cause intoxication or other behavioral effects), they must
know when to take the blood samples. Accordingly, they also must know when
the greatest alcohol consumption occurs, because BACs are likely to be highest
shortly afterwards.
For
example, Jelic and colleagues (1998) compared two mouse strains called CBA
and TO that received an alcohol-containing liquid diet2 (2 The diet contained 3.5-percent
alcohol (comparable to the alcohol content in beer) for 2 days, then 7-percent
alcohol for 5 days.) as their only source of food and were kept under
the same environmental conditions. The investigators found that the CBA mice
exhibited their peak BACs around 7 p.m. and that BACs remained relatively
high until 5 a.m. (see figure 1). Conversely, for the TO mice the peak BACs
were observed at 9 a.m. and dropped rapidly afterwards. Thus, to determine
peak BACs, researchers would need to take blood samples from the animals at
different times of the day.
Differences
in the Type and Administration Schedule of the Alcohol-Containing Diet
The
approach that researchers choose to motivate study animals to consume alcohol
(and the alcohol concentration in the respective fluids) can substantially
alter overall alcohol intake and diurnal consumption patterns. For example,
Jelic and colleagues (1998) compared the alcohol consumption patterns (and
resulting BACs) of CBA and TO mice that were maintained either on a diet consisting
of regular laboratory food plus alcohol solution as the sole fluid or on an
alcohol-containing liquid diet. The investigators found that animals of both
strains consumed less alcohol (and therefore experienced lower BACs) when
they received the alcohol in their drinking water. Moreover, when the alcohol
was provided in the drinking water, fluid consumption was more evenly distributed
throughout the day and/or peaks were lower and occurred at different times
than when the alcohol was part of a liquid diet (see figure 1).
Figure 1 Alcohol consumption patterns of two mouse strains (CBA and TO) that received alcohol either in a liquid diet containing 7-percent alcohol that constituted the animals sole source of food or as an alcohol solution in addition to regular laboratory food. When the alcohol was provided in the form of a liquid diet (circles), the alcohol consumption patterns (and thus the blood alcohol concentrations [BACs]) of the two mouse strains differed considerably. In the CBA mice (A), BACs peaked around 7 p.m. and remained high throughout the night. Conversely, the BACs in TO mice (B) showed a sharp peak around 9 a.m. When the alcohol was provided in the form of a solution containing 10 percent (empty squares) or 20 percent (half-empty squares) alcohol, however, alcohol consumption and BACs in both strains were consider-ably reduced and distributed more evenly throughout the day. Furthermore, no substantial differences existed in the alcohol consumption patterns of the two strains under these conditions. (Error bars are not shown.)
mM = millimolar.
SOURCE: Jelic et al. 1998.
The concentration of the alcohol solution offered also can influence consumption patterns. In one study, rats were offered alcohol in increasing concentrations ranging from 2 to 10 percent in their drinking water (Boyle et al. 1997). The animals had access to the drinking solution for 23 hours per day, after which it was withheld for 1 hour. Slight variations in the circadian consumption patterns existed with different alcohol concentrations. Thus, with most concentrations, the animals consumed a relatively large amount of alcohol in the hour after they regained access to the alcohol solution. With the 6-percent alcohol solution, however, the animals drank a substantially smaller amount during that first hour than with the other alcohol concentrations. Furthermore, whereas with most alcohol solutions the majority of the consumption occurred during the dark phase, a relatively high level of consumption occurred during the light phase with the 2-percent solution. Finally, the number and intensity of consumption peaks varied for different alcohol solutions (see figure 2).
Figure 2 Alcohol consumption patterns of rats receiving drinking water containing (A) 2 percent, (B) 6 percent, or (C) 10 per-cent alcohol. The animals had access to the alcohol solution for 23 hours per day. Both overall fluid consumption and circadian consumption patterns differed depending on the alcohol concentration in the drinking water. The horizontal dark line indicates the dark period in the animals light-dark cycle. (Error bars are not shown.)
mL= milliliter.
SOURCE: Boyle et al. 1997.
These
observations indicate that the concentration of the alcohol solution offered
can influence circadian consumption patterns in rodents.
Disruption
of the Normal Circadian Rhythm
Other
investigators studied the effects of a disrupted circadian cycle by offering
alcohol solutions to rats kept under constant light or constant darkness (Goodwin
et al. 1999). This analysis found that both during an acquisition phase, in
which the animals became accustomed to alcohol, and during a subsequent maintenance
phase, in which the animals had constant access to alcohol, exposure to constant
light suppressed the animals alcohol consumption compared with animals
kept under normal lighting conditions. In contrast, exposure to complete darkness
during both the acquisition and the maintenance phase did not substantially
affect the rats alcohol consumption compared with animals kept under
normal lighting conditions. Furthermore, the different lighting conditions
did not influence the animals water consumption. These findings suggest
that certain lighting conditions can specifically affect alcohol intake.
The influence of a disrupted circadian rhythm with altered lighting conditions was further investigated by Gauvin and colleagues (1997), who altered the lighting conditions of rats in a manner that reflected the jet lag experienced by travelers crossing several time zones and the repeated changes in schedule experienced by shift workers. In those experiments, animals exposed to the jet lag conditions showed temporary increases in alcohol consumption. Moreover, animals kept under shift work conditions demonstrated significantly increased alcohol intake over a 2-month testing period. These findings further support the notion that a disruption of the normal circadian rhythm can result in increased alcohol consumption.
Concurrent
Administration of Other Drugs
Choi and colleagues (1990) speculated that among other mechanisms, brain chemicals called endogenous opioids may play a role in the relationship between morphine administration, disruption of the circadian cycle, and alcohol consumption. The activity of these substances changes across the daily cycle, and disruptions in the circadian rhythm may perturb the functions of the endogenous opioids. Morphine is chemically related to the endogenous opioids and therefore also affects brain functions controlled by the opioids. Consequently, morphine administration may exacerbate the effects of a disrupted circadian cycle on endogenous opioid function.
Signaling
Systems in the Body
The bodys endogenous signaling
systems, such as certain hormones or neuropeptides--that is, small protein
molecules that relay information to nerve cells--also control alcohol intake.
One such hormone and neuropeptide called cholecystokinin (CCK) is released
in the gastrointestinal tract and brain and appears to serve as a signal of
satiety. For example, researchers have demonstrated that exogenous CCK suppresses
feeding and alcohol intake in rodents (Kulkosky et al. 1991).
The magnitude of this effect appears to be dependent on whether CCK is administered and alcohol is available during the dark phase or the light phase of the daily cycle. To demonstrate this interaction between CCK and the lighting cycle in reducing alcohol intake, Kulkosky and colleagues (1991) used a limited-access procedure in which rats had access to alcohol for only 40 minutes per day either during the dark phase or during the light phase. Immediately before each access period, the animals were injected with CCK. The investigators found that CCK limited alcohol intake by a greater proportion compared with the control animals when CCK administration and alcohol access occurred during the dark phase than when they occurred during the light phase. These observations are consistent with those of other researchers who noted that another gastrointestinal neuropeptide called bombesin also appeared to inhibit feeding more potently at night (Kulkosky et al. 1991). However, for both bombesin and CCK, the details of the experimental design appear to influence to a certain extent whether they are more effective during the dark or during the light phase (Kulkosky et al. 1991).
Properties
of Light
One line of research into the
factors that influence the relationship between alcohol consumption and lighting
conditions has addressed the issue of whether the wavelength of light used
plays any role in this relationship. These studies were triggered by observations
that rats whose cages were placed by a window had a greater preference for
alcohol than did rats whose cages were placed at other locations in the laboratory
(Wilson et al. 1976). Because initial studies detected no effect of light
intensity (which would be greater by the window) on alcohol consumption, Wilson
and colleagues (1976) investigated the effect of ultraviolet (UV) light (the
levels of which also should be higher near the window) on alcohol preference.
In
that experiment, rats were kept under diffuse white light for 12 hours per
day for 10 days before one-half of the animals were placed under UV light3
(3 The wavelength of the ultraviolet
light (i.e., 350 nm) was such that it was not harmful to the animals in any
way.) for 12 hours per day for 30 days. Within 3 days of placing the
animals under UV light, their alcohol consumption increased markedly, accompanied
by a substantial decrease in water consumption. This increase in alcohol consumption
was not permanent, however. When the UV light was removed, the animals
alcohol consumption slowly decreased to its normal levels over a period of
30 to 40 days. Conversely, the alcohol and water consumption of the control
animals remained stable throughout the experimental period. The mechanism
through which UV light exerts its effect on alcohol preference still remains
unclear, although it is known that UV light potentiates the toxicity of other
ingested chemicals.
Conclusions
Researchers have studied the relationship between the light-dark cycle and alcohol consumption for approximately 30 years and have uncovered solid evidence that at least in rodents, the lighting conditions influence normal drinking behavior. Furthermore, scientists have identified several factors that modulate this relationship. Despite these extensive investigations, however, the field is still evolving and many questions remain. For example, the role of the pineal gland and its hormone melatonin in governing alcohol consumption has not been determined conclusively. Other topics, such as the properties (e.g., wavelength) of the light that may play a role in regulating alcohol consumption, have not yet received the attention they deserve.
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