Engaging in resistance exercise can increase slow wave sleep percentage without disrupting the other components of sleep and learning. Low levels of serotonin are usually related to early slow wave sleep, so those with higher REM sleep early on may have lower levels. Sleeping for full ultradian cycles may be better than waking up in between cycles as disruptions may impact the sequencing of slow wave and REM sleep.
The second half of sleep is enriched in rapid eye movement (REM) sleep, which is beneficial for intense dreams. Serotonin precursors like 5-HTP or L-tryptophan are not recommended for sleep as they can cause difficulty falling back asleep and disrupt the natural sleep cycle.
The use of THC may disrupt REM sleep, leading to a rebound effect of increased REM sleep when usage is stopped. Alcohol is also shown to negatively impact the quality of sleep.
Melatonin plays a major role in controlling the body's metabolic or circadian rhythm. While the pineal gland produces melatonin, taking too much of it can reduce its natural production.
This podcast explains the different stages of sleep including non-REM sleep (which has three stages that become progressively deeper) and rapid eye movement (REM) sleep. Humans spend more time in REM sleep than any other primate, but sleep less overall than other primates.