Have you ever
noticed how some people are able to wake up at 5 in the morning to work out, while
others prefer midday or late night gym sessions? Researcher may have found
clues as for the best time of the day to work out.
The mechanisms
and function of sleep are still widely unknown. However, sleep has been
shown to be controlled by circadian rhythms and homeostatic processes, as
described in a lecture by Dr. Cavanaugh on circadian rhythms and sleep gating
in Drosophila. The circadian mechanism regulates the timing of sleep and
the homeostatic processes regulate both the amount and intensity of
sleep. Circadian rhythms are modulated by internal clock mechanisms
including different proteins that control the transcription of genes into
proteins involved with circadian regulated behaviors, namely sleep-wake
behaviors. Due to these internal mechanisms, humans can still produce circadian
molecular products to regulate sleep-wake behaviors around 24 hours even when
put into caves without day and night transitions. Those produced proteins act
on the brain, the liver, and other parts of the body to control behavior. In
humans, the internal clock is the suprachiasmatic nucleus.
Circadian
rhythms can also anticipate and adapt to external environment. Circadian
rhythms can become disrupted by exposure to light at night, the amount of sleep,
and changes in sleep patterns. A good example of a disruption in circadian
rhythms is jet lag. This occurs when a person moves to a different time zone,
effectively shifting their sleep schedule and resulting in poorly timed
circadian rhythms because their body is not used to the external
environment. Unfortunately, disrupting sleep cycles and circadian rhythms
can lead to larger health consequences, such as certain types of cancers, mood
disorders, obesity, and diabetes. The circadian internal clock mechanism
also becomes less functional with age.
Homeostatic
mechanisms increase sleep pressure with the amount of time spent awake. To
study sleep behavior in animals and humans, many researchers use EEGs. In the deepest
part of sleep, the brain emits delta waves, which are waves with large
amplitudes. According to Crocker and Seghal, increased power in the delta waves
during deep sleep indicates higher homeostatic drive, showing researchers the
level of sleep pressure on an animal.
In a study performed
by Schroeder et. al., mice with a functional circadian clock systems and mice
without a functioning internal clock were examined to study if exercise during
a certain time of day can effectively regulate circadian rhythm or repair a
broken clock. During their first experiment, healthy mice were either
given access to run on their wheel anytime, given access only during the
afternoon equivalent for mice since they are nocturnal animals, or given no
time to run. They did not see much difference between the two running mice, but
the running mice produced many more proteins in clock cells compared to the
mice not given access to run. When testing mice without functioning internal
clock mechanisms, after a few weeks of running, their circadian clock products
were more likely to reach their targets, such as the heart, liver, and other
regions of the brain, to control sleep-wake behaviors. Researchers saw that
mice that ran earlier in the day produced less proteins in their internal clock
cells than mice who ran later in the day, suggesting that the optimal time of
the day to exercise is in the afternoon.
This theory fits
well with ideas presented in the lecture given by Dr. Cavanaugh based on the
competing sleep systems. The circadian and homeostatic systems are in
competition with one another. As homeostatic begins to rise in afternoon,
circadian mechanisms also rise to prevent premature sleep timing, counteracting
the effects of homeostatic sleep drive. Schroeder et. al. concluded that
exercising helps to regulate the production and function of internal clock
mechanisms that regulates sleep-wake behavior, but researchers cannot definitively
state when the best time to work out is. The researchers advise against
going to the gym very late at night, but they do not distinguish between an
early morning and an afternoon workout.
References:
Cavanaugh,
Daniel J., Abigail S. Vigderman, Terry Dean, David S. Garbe, and Amita Sehgal.
"The Drosophila Circadian Clock Gates Sleep through Time-of-Day Dependent
Modulation of Sleep-Promoting Neurons." Sleep 39.2 (2016): 345-56.
Web. 28 Feb. 2017.
Crocker, A., and
A. Sehgal. "Genetic Analysis of Sleep." Genes & Development
24.12 (2010): 1220-235. Web. 28 Feb. 2017.
Reynolds,
Gretchen. "Why Afternoon May Be the Best Time to Exercise." The
New York Times. The New York Times, 12 Dec. 2012. Web. 28 Feb. 2017.
Schroeder,
Analyne M., Danny Truong, Dawn H. Loh, Maria C. Jordan, Kenneth P. Roos, and
Christopher S. Colwell. "Voluntary Scheduled Exercise Alters Diurnal
Rhythms of Behaviour, Physiology and Gene Expression in Wild-type and
Vasoactive Intestinal Peptide-deficient Mice." The Journal of
Physiology 590.23 (2012): 6213-226. PubMed. NCBI. Web. 28 Feb. 2017.
