Sleep is a natural process that allows the body and brain to rest and recover, and it involves changes in brain activity, physiology, and behavior that serve various functions and benefits for health and well-being. ^{1 9 3}

Some of the basic concepts related to sleep are:

• Sleep phases: Sleep can be divided into two major phases: non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. Each phase has distinct characteristics and functions in the sleep cycle. ^{9 3 9}
• Sleep cycle: A sleep cycle is a period of time that includes different stages of sleep. Each cycle lasts about 90 to 120 minutes and repeats four to six times per night. The stages of sleep are NREM stage 1, NREM stage 2, NREM stage 3, and REM stage. ^{1 3}
• Circadian rhythm: Circadian rhythm is the body's internal clock that regulates the timing of sleep and wakefulness. Circadian rhythm is influenced by light exposure, hormones, and other factors. ^{1 2 4}
• Sleep functions: Sleep affects nearly every aspect of our physical and mental health, from mood and memory to immunity and metabolism. Getting enough quality sleep is vital for our overall well-being and functioning. ^{11 13 12 4}