Slow-wave sleep at the beginning of the night is more critical for health than REM sleep later in the night.

“Yeah, REM sleep is toward the end of the night. That's when you dream, and that's also a good part of sleep as well, but nothing beats slow-wave sleep with these large delta waves if you were to look at this on a polysomnography.”

“So the most important time of sleep is the sleep that you get at the beginning of the night. There's really two types of sleep that are the best at making you feel healthy. And they are slow-wave sleep, which is right at the beginning of the night. In fact, this is the type of sleep that is associated with growth hormone secretion, especially in children. If there was ever the fountain of youth, this would be it. Growth hormone makes you feel younger, makes you look younger.”