There are two major states of sleep, non-REM and REM. Non-REM sleep has three phases, each characterized by different levels of brain activity, while REM sleep is associated with vivid dreaming and rapid eye movement. A perfect night's sleep involves cycling through these phases multiple times.
Although oversleeping is often linked to other health issues, such as sleep apnea or depression, consistently not getting enough sleep can also have negative effects on cognitive function.