From the March 31 issue of The Lancet:

The ease of getting to sleep and staying asleep depends not only on previous wake time, but also on associations with the circadian rhythm of core temperature. Sleep is easiest to initiate when core temperature is falling rapidly or is at its lowest, and most difficult when body temperature is rising rapidly or is high. Waking is the opposite of sleep initiation, because it happens when core temperature is rising or is high.

Normally, our body is at its coolest in the wee hours of morning, just before 5 a.m. Our body temperature peaks around noon, when we are most awake. I think it’s interesting that, as this Lancet article suggests, body temperature might provide a crucial somatic signal for sleepiness. (So don’t go to the gym and get sweaty right before bedtime. The brain associates heat with wakefulness.) This is yet another example of the William James/Antonio Damasio hypothesis, which is that our flesh plays a crucial role in many different aspects of cognition, emotion and decision-making. This sleep/body temperature theory also jives with my own sleep habits. Much to the chagrin of my girlfriend, I like sleeping in cold conditions. I’ll leave a window open in the middle of winter and turn down the heat so that our home resembles a refrigerator. We don’t have an air-conditioner, but if we did I’d set it on 55 degrees. I thought this was a personal quirk, but now I know that the cooler temperatures are just preparing my mind for sleep. I love when science justifies my habits.