The stages of sleep include three non-rapid eye movement (NREM) stages and one rapid eye movement (REM) stage. The stages proceed in the following order and cycle such that N1 follows REM: N1, N2, N3, REM.

During N1, the individual loses sensation of his or her environment and becomes more relaxed, both mentally and physiologically. Brain waves slowly transition from alpha waves (characteristic of wakefulness) to lower frequency theta waves.

During N2, environmental awareness completely disappears. Brain waves are characterized by sleep spindles and K-complexes, which are short bursts of higher frequency waves. General wave patterns follow theta wave trends, with the exception of these interruptions.

During N3, the individual enters deep sleep. Brain activity is characterized by a high presence of very low frequency delta waves. Parasomnias, such as sleepwalking and night terrors, can occur during stage N3 sleep.

REM sleep is characterized by neural acetylcholine secretions, which cause brain waves to increase in frequency and resemble alpha forms. The body's muscles are paralyzed, though the brain's activity is elevated. Dreams occur during this stage, and muscle paralysis is thought to prevent the sleeping individual from attempting to interact with their dreams.

Delta waves are most concretely linked to the N3 stage of sleep.

The non-rapid eye movement stage of sleep, NREM or synchronized sleep, involves four stages. The transition from wakefulness to sleep occurs during stage 1. Eye movements are slow and the electroencepalogram (EEG) shows low brain wave activity. In stage 2, EEG activity is increased. Spikes called K complexes are recorded. In stage 3, eye movement ceases. Wave frequency is reduced and amplitude is increased. Delta activity is recorded on the EEG in stage 4. Stages 3 and 4 are considered deep sleep.

Stages 3 and 4 of non-rapid eye movement sleep (NREM sleep) are considered deep sleep. Stage 3 is characterized by an increase in amplitude and a reduction of wave frequency. During stage 4 sleep, delta activity is recorded. Stage 1 is the transition from wakefulness to sleep. The EEG shows low brain wave activity. In stage 2, EEG activity is increased, with the appearance of spikes called K complexes.

Sleep has two states: one with no rapid eye movement (NREM or synchronized sleep, which involves four stages) and one with rapid eye movements (REM or dreaming sleep). NREM sleep comprises approximately 75% of the sleep cycle. During NREM sleep, respiration becomes slower and more regular as transition from wakefulness to sleep occurs. During stage 4, or deep sleep, delta activity is recorded on the EEG.

During rapid eye movement sleep (REM sleep), dreaming can occur. Blood flow to the brain is increased and breathing is more irregular. Penille erections may occur. The transition from wakefuness to sleep occurs during non-rapid eye movement sleep (NREM sleep). 

In rapid eye movement (REM) sleep, an electroencephalogram (EEG) shows low brain activity and muscle paralysis normally occurs. During non-rapid eye movement sleep (NREM), stage 1 eye movements are slow. EEG activity increases during NREM stage 2. Eye movement ceases during NREM stage 3. Delta wave activity occurs on EEG readings during NREM stage 4. NREM stages 3 and 4 are considered deep sleep.

During non-rapid eye movement (NREM)sleep, when the transition from wakefulness to sleep occurs, the pulse becomes slower and more regular. During rapid eye movement sleep (REM sleep), dreams occur, blood flow to the brain increases, breathing is more irregular, metabolic rate increases, and muscle paralysis occurs.