By Karthika Swamy Cohen
Does temperature affect sleep patterns? Studies have shown
that factors such as room temperature, as well as core body and distal skin
temperatures, can have a significant impact on the length and frequency of REM
bouts during sleep.
At the “Advances in Modeling Sleep-wake Behavior and Circadian
Rhythms” minisymposium on Wednesday at the SIAM Conference on the Life Sciences,
Pamela Pyzza (Ohio Wesleyan University) described mathematical modeling of
influence of temperature on human sleep patterns.
Thermoregulation is the process by which our body maintains
its core internal temperature. When at a cold location, our body temperature
tends to slowly move towards the ambient temperature. The body cools down or
heats up to maintain a nice middle ground. Thermoreguation is at play at certain times during
sleep. It occurs during non-REM (NREM) sleep but not during REM sleep.
The body tries to raise its internal temperature during
sleep by awakening, changing positions, blood vessel contraction and
vasodilation, and by increasing NREM sleep (when it can be thermoregulated).
Pyzza and her group modeled the homeostatic process along
with the circadian process in order to factor in the influence of temperature.
sleep-wake model incorporates details of both ambient and body temperatures.
Morris-Lecar equations are used to describe the mean
activity of each neuron population involved in the sleep wake cycle. Neurons in
the preoptic anterior hypothalamus (POAH) are active in sleep and those in the midbrain
reticular formation (MRF) are active during wake times.
The model follows eight hours of sleep followed by a 16-hour
wake cycle. There are 4-5 REM and NREM cycles per night, the formed being
smaller than the latter. The model falls asleep into NREM and wakes up from
Temperature effects play a role in two places during the
cycle, and these are described mathematically by differential equations. In REM
sleep, we don't thermoregulate, so we just tend to ambient temperature: this
results in a smaller differential equation.
Humans can maintain a steady temperature within a range in
which our body can thermoregulate. If it’s drastically cold or drastically warm
(outside of this range), the body just tends toward ambient temperature.
When away from a comfortable ambient or thermoneutral
temperature (about 29 degrees centigrade), the model shows that it doesn’t take
a significant amount of time to fall asleep, there is an increased amount of
wakefulness. This trend in sleep behavior is consistent with experimental work
for sleep at drastic ambient temperatures.
The group also used the model for jet lag simulation. Here,
they found that forcing the system to fall asleep five hours later—as in, after
a long flight—instigates long REM latency and higher REM homeostat. This clearly
indicates a shorter night sleep during the first night. It shows a longer REM
latency period at the start of the following night and a greater amount of REM
by the end of the night.
At a related minisymposium on Tuesday, “Dynamical Systems with Applications to Biology and
Medicine,” Selenne Bañuelos (California State University) discussed a mathematical
model that studies the effects of temperature on REM/nonREM dynamics. The human sleep–wake regulation model she
described, which is based on previously developed models of sleep dynamics and
thermoregulation, simulates increased duration and number of REM bouts through
the night and appearance of awakenings resulting from digressions in
thermoneutrality in the body. The model demonstrates the importance of
temperature to sleep regulation.