CHAPTER 20Rhythms, Sleep, and Dreams
Many (perhaps most) animals have daily periods of inactivity and
reduced alertness. For us humans and other vertebrate animals, there
are distinctive physiological properties of brain activity that charac-
terize this period of reduced activity. We call this period of time sleep.
The amount of time spent asleep each day varies substantially
across different species of animal. Here are a few approximate sleep
times for several mammals, in hours per twenty-four-hour day: bat,
19; mouse and rat, 12; rabbit, 9; cow and elephant, 4. Typically, we
humans spend eight hours asleep out of each twenty-four-hour day
—one-third of our lifetime. If we live to be ninety years of age, that’s
a full thirty years spent asleep, amounting to 263,000 hours—a lot of
time. (At the very least, this argues for having a comfortable bed.)
If someone is tired and is deprived of sleep, the desire for sleep will
become overwhelming, taking precedence over all other desires. That
sleep has been preserved throughout hundreds of millions of years of
biological evolution suggests that it serves very important functions.
And although the scientific study of sleep has blossomed over the
last several decades, most of what is going on during sleep remains a
mystery.