Slaap-waakcycli

Sleep is a physiological state of partial consciousness with reduced sensory and motor activity that can be disrupted by external stimuli.

The sleep cycle is made of periods of non-rapid eye movement, or NREM sleep, and rapid eye movement or REM sleep.

NREM is characterized by deep sleep with reduced respiratory and heart rates, no dreams, and limited body movements. 

NREM sleep usually lasts around 90 minutes, after which the person goes into REM sleep.  

REM sleep is marked by elevated heart and breathing rates and is the stage where most dreams occur. During REM, skeletal muscle movements are inhibited, but the eye muscles are not. They rapidly move back and forth.

The early cycles of REM sleep are brief, lasting an average of 5 minutes, and gradually increasing to 20 to 50 minutes over an 8-hour sleep period.

 Sleep and wakefulness are regulated by the reticular activating system or RAS, a network of neurons located in the brainstem.

Sleep occurs when hypothalamic neurotransmitters inhibit the RAS, while arousal happens when stimuli like touch, light, or sound activate the RAS.