While experts are testifying about the details of Michael Jackson's REM sleep cycles in the trial against the star’s concert promoter, AEG Live, the definition of quality sleep may also be scrutinized.
The recommended sleep for most adults is seven to nine hours per night, according to the National Sleep Foundation. But that doesn't take waking time during the night into account, and it doesn't specify how much time is ideally spent in each cycle. A better definition would include sleep quality and efficiency, instead of hours in bed, said Chris Berka, CEO and founder of Advanced Brain Monitoring.
"A complete sleep cycle typically takes about 90 minutes and the rule of thumb is that you need 4-5 full sleep cycles," Berka said. "But there's no evidence that all 7-9 hours have to occur in a single bout."
A sleep cycle is comprised of four stages of non-REM sleep followed by REM, or dreaming, sleep. The first cycle is such a light phase of sleep -- think of it as the bubble between wakefulness and sleep -- that some expert believe it has no recuperative value.
But the other three stages each offer unique -- and necessary -- restorative properties. Stage 2 can restore alertness; it "gives you energy to move forward," Berka said. Many experts target stage 2 as ideal for napping.
Stages 3 and 4, also called slow wave sleep, are related to enzyme regeneration, rebuilding proteins, healing and immune functions. If you wake people in the middle of one of these stages, they will likely feel groggy. REM sleep is not fully understood, but it seems to impact emotions and memory.
"You can REM-deprive people for long periods of time, but their memory is terrible," Berka said. "And you may not dissociate emotions from your memory."
Despite the unknowns, Dr. Nancy Wesensten of the Walter Reed Army Institute of Research, who studies sleep in order to help soldiers optimize their rest, says it’s actually fairly simple to quantify quality sleep.
"When people talk about low-quality sleep, what they really mean is sleep that is interrupted by wakening," Wesensten said. "What awakenings do is they reduce the amount of sleep time. And in some people's view, including mine, that frequent disruption causes you to transition to the lightest stage and you don't include that in recuperative sleep. So it's something you can measure directly: Quality sleep is the amount of time which is set aside for sleep spent in the recuperative stages."
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.
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.
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."