MadSci Network: General Biology

Re: why do people get cold when they sleep

Date: Wed Mar 6 17:50:49 2002
Posted By: Bernadette Baca, Health Physicist, Division of Reactor Safety
Area of science: General Biology
ID: 1010508575.Gb

Why do we as humans, complex and intelligent mammals, get cold when we 
sleep?  This question was answered through the many studies of sleep.  As 
researchers studied "sleep", they were able to uncover many secrets of the 
human body and brain.  Everyone knows that sleep is essential to rest and 
heal the body and mind.  Basically the human being has two internal clocks 
that help regulate wakefulness and sleep: circadian rhythm and the 
body-temperature clock.  The circadian rhythm, also known as the sleep 
clock, has been the most studied of the two biological clocks and found to 
be linked to the body-temperature clock.  The circadian rhythm is 
influenced by the amount of light and darkness we experience during the day 
and provides the first signals to begin sleep.  The body-temperature clock, 
however, is not influenced by external stimuli and is seen as the most 
primal of our internal clock.

During the day, a healthy body's temperature only varies by about a degree 
or two Fahrenheit (half a degree or a degree Celcius).  Interestingly, the 
low points of body temperature cycle correspond to "sleep gateways", times 
when it is easy for people to fall asleep.  Typically the two lowest points 
of body temperature are in late afternoon (3-6 pm) and early morning (3-6 
am) with the highest values commonly occurring in the evening (7:00 p.m. to 
10:00 p.m.).  Studies in which the body temperature has been monitored in a 
time-free environment have shown that our temperature level fluctuates in 
the same 25 to 26-hour pattern, no matter when we sleep or when we are 
awake.  In short, our body temperature cycle operates independently of our 
sleep/wake cycle. The early morning period our body temperatures are much 
lower; thus why we may feel cold waking up some mornings.  In the same way, 
the periods of highest body temperature are directly linked with an 
inability to go to sleep.  By the way, studies have shown that exercise has 
no effect on the body temperature clock.

The two clocks reinforce each other.  The sleep clock generally tells the 
person to go to sleep just as his body temperature is starting its nightly 
decline.  If the person ignores it, or has thrown off his sleep clock, the 
body temperature clock comes around and virtually forces him to go to sleep 
between 4 and 6 am.  Then exposure to daylight when the person awakens 
resets the sleep clock.

The documents this response is based and for any additional information on 
the physiology and anatomy of sleep can be found in the following 

"How the Human Brain Developed and How the Human Mind Works" by Manfred 

"Circadian Rhythms - Critical Concepts" by Round-the-Clock-Systems:

"Frequently Asked Questions about Training Nutrition - Part Five - 
Self-assessment - Section 8" Version 6.6 (2/3/97)by Paul Moses:

The Sleep Clinic:

The Sleep Medicine Home Page:

The Natural Health School:

The Brain Web:

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