Your sleep-wake cycle is determined by your inner clock and your sleep pressure, while skin temperature is an important sleep-permissive factor.


Scientists think that your sleep-wake regulation depends on your circadian pacemaker (= inner clock) and your drive for sleep or sleep pressure. The latter builds up while you are awake and dissipates during sleep, and is therefore sometimes described as the hourglass of the brain.

In real life however, we all know that this is not enough to make sure that you have a good night’s sleep. There are other conditions, such as lying down and a dark room, that are crucial.

Skin temperature changes with many of these factors, and is thus an instrument the brain uses to know if it is ok to sleep. Scientists consider it therefore as a sleep-permissive factor.

### Circadian rhythm and sleep pressure

Your circadian rhythm is determined by a small structure deep in your brain called the suprachiasmatic nucleus of the hypothalamus. It also modulates the rhythm of your core body temperature and the secretion of some hormones such as melatonin and cortisol. You constantly adjust and refine it with the help of signals from the environment, such as the night/day cycle.

Core body temperature increases during the morning and reaches a peak in late afternoon, and then it decreases again to have its low point during the second half of the night. Melatonin secretion follows the inverse time course.

The circadian rhythm and the sleep pressure complement each other: when your sleep pressure increases at the end of the day, your circadian rhythm keeps you awake, and when during the night the sleep pressure dissipates, the circadian rhythm allows you to sleep on.

The result is that healthy people fall asleep while their core temperature is going down and their melatonin secretion increasing. They normally sleep during the maximum of the melatonin secretion and the low point of their body core temperature cycle, and wake up while their core temperature is rising but still low and their melatonin level high but falling.

### Skin temperature

Even if you have enough sleep pressure and are at the right moment of your circadian rhythm, you are not always able to sleep. Scientists have noticed that a rise in skin temperature is necessary to fall asleep. A number of factors, including lying down in a dark place, promote a dilatation of the blood vessels in your skin, which increases its temperature. We are obviously talking here about very small differences in temperature, often as low as 0.4 C, which are not stressful for your body, since nobody can sleep well in uncomfortable temperatures.

The reverse seems to be true as well: a lower skin temperature makes you more alert.

### Practical implications

An increased skin temperature is a signal to the brain that it is fine to sleep. However, scientists have noticed that in some people this signal does not work very well, and that a gentle warming-up of the skin can improve their sleep quality.

If you want to know if that is your case, you can use a skin thermometer and compare the readings with your sleep quality. You could also just experiment with it, even though it is not as easy as it sounds, as you have to warm your skin slightly while allowing your body core temperature to go down.

-You can pre-warm your bed using a heating blanket. Switch it off during sleep though, because it could increase your core temperature.

-Take a warm foot bath before bedtime or use bed socks.

-Have a warm drink.

-Take a sauna, and wait until your core body temperature has recovered. Your skin temperature will remain elevated for a little longer, which gives you a window during which it is easier to fall asleep.

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**References**

Derk-Jan Dijk, Steven W. Lockley. Invited review: Integration of human sleep-wake regulation and circadian rhythmicity. J Appl Physiol; 92(2): 852-862

Kurt Krauchi, Anna Wirz-Justice. Circadian clues to sleep onset mechanisms. Europsychopharmacology; 25(S5): S92-96

Roy J.E.M. Raymann, Dick F. Swaab, Eus J.W. Van Someren. Skin deep: enhanced sleep depth by cutaneous temperature manipulation. Brain; 131(2):500-513

Nico Romeijn, Roy J.E.M. Raymann, Els Most et al. Sleep, vigilance and thermosensitivity. Pflugers Arch; 463(1):169-176