Slow-wave sleep and REM sleep occur in different proportions throughout the night, with slow-wave sleep dominating early cycles and REM sleep dominating later cycles.

“So early in the night, these 90-minute cycles tend to be comprised more of shallow sleep and slow wave sleep. And we tend to have less so-called REM sleep, R-E-M sleep, which stands for Rapid Eye Movement sleep. For every 90-minute cycle that we have during a night of sleep, we tend to start having ”

A primary purpose of deep sleep is to allow mitochondria to rest and restore the energy reserves used during the day.

“I think with dreaming, there's obviously other things going on there, but deep sleep, the one of the primary things that's happening is you need to give your mitochondria rest”