What Is Good Sleep? - 2

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A complete sleep cycle typically takes about 90 minutes, said Berka, but there's no evidence that it all has to happen in a single bout. Circadian rhythms may make it difficult to sleep during the day, but some experts believe that humans haven't always slept eight hours in a row.

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When psychiatrist Thomas Wehr of the National Institute of Mental Health experimented with the sleep cycles in the '90s, he found that when participants were given a 14-hour period of rest time, most of them divided their sleep into two distinct blocks, separated by an awake period of one to two hours. Later, Historian A. Roger Ekirch noted references from before the Industrial Revolution to "first" and "second" sleeps.

“If we didn't have artificial light, we would be inclined to get more sleep,” Wehr said.

Sleep Habits Vary by Race

Increasing evidence also shows that quality sleep varies dramatically from person to person, Berka said.

"The average person needs seven to eight hours, but within that, there are short sleepers and long sleepers. As many as 15 percent of the population thrive on less sleep -- four to five hours a night, and on the other side, long sleepers need nine."

Quality of sleep may even depend on the quality of wakefulness, a study published last week suggests. When researchers kept mice awake in two distinctly different ways, they found that each group experienced different biochemical changes.

Part of our culture’s current relationship with sleep, some say, is that we prefer to be awake.

"The reality is, the world we live in, it's just hard to get enough sleep," Wehr said. "I'm like everybody else: I don't get enough. We're addicted to wakefulness... .We don't like to stop what we're doing and go to sleep."