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."