National Institutes of Health
Body temperature normally drops about 2 degrees during sleep.
We are typically most alert when our body temperatures are highest, and most sleepy when our body temperatures begin to drop. This is one reason why a cool room is conducive to a better night’s sleep, and why night sweats or the inability to cool off is associated with difficulty sleeping.
The natural cooling we experience during sleep is often reduced as we age. While the body temperature of a healthy young adult may drop about two degrees during sleep, by age 75 some of us may experience a drop of only around a half degree.
This reduction in the amount of body cooling, combined with a faster than normal biological clock, can result in a significant flattening of the overall circadian rhythm as we age. For insomniacs, this flattening can be manifested by more awakenings during the night – characterized by lighter, more fragmented sleep – and by less alertness during normal waking hours – as characterized by more napping during the day.
So as we age there may be a tendency to spread out sleep more evenly over each 24-hour period, rather than having two sharply defined periods of sleep and waking.
There are a number of effective methods to counter this age-related tendency toward a flatter circadian rhythm. Doing so helps produce more robust sleep and supports greater alertness and energy during waking hours. These are worthy goals, and the STS will help you achieve them.
A cool room is just one of many ways you can help yourself sleep better. Using a disciplined sleep schedule, controlling negative sleep thoughts, reducing chronic stress, and creating an optimal sleeping environment all work together to help counter a flattening circadian rhythm and support better sleep. In the STS, we will examine all these topics in detail.
The biological pressure to sleep cannot be resisted
After about 16 hours of nonstop wakefulness, we normally feel a biological pressure to sleep. This pressure, known as the homeostatic sleep drive, is controlled by a separate neurological regulatory mechanism in addition to the biological clock.
This regulatory mechanism is keyed by a pinhead-size cluster of brain cells known as the ventrolateral preoptic nucleus, or VLPO. The VLPO is sensitive to a chemical naturally produced in the brain known as adenosine, one of several neurotransmitters involved with our daily sleep-wake cycle. After about 16 hours of adenosine build-up, the VLPO sends out a signal that it’s time to sleep.
In this way, the brain essentially keeps track of how long it is awake, and after about 16 hours reinforces the circadian rhythm to induce sleepiness. For optimal sleep, it's therefore important the two processes -- sleep drive and the biological clock -- are synchronized and working well together. Later in the STS, you will learn how to enhance this mutually supportive process that can make sleep when you want it practically irresistible.
As we age, the VLPO typically loses some of its neurons, and thus some of its ability to drive sleep. This is one reason why as we age we tend to experience lighter, more fragmented sleep. So some diminishment of the VLPO is therefore a normal consequence of aging, and this helps explain why the elderly often sleep less than younger adults.
To counter this natural age-related reduction of the VLPO, there are a number of simple, effective, completely drug-free methods you can use to strengthen and in effect rev up your sleep drive. We will cover this in the first week of the STS.
Interestingly, caffeine apparently disrupts the neurological homeostatic process by blocking the VLPO’s ability to recognize adenosine. So in addition to being a metabolic stimulant, this is another reason why caffeine can disrupt sleep.
Despite the unknowns however, this is a certainty: the longer an individual remains awake, the stronger the desire and need to sleep becomes.
We call this irresistible certainty the “Law of Prior Wakefulness”, and we will use this principle extensively in the STS.