Function of Sleep
What is sleep?
A naturally occurring ASC that we all experience every day. Sleep is an ASC that features the suspension of awareness of the external environment and is accompanied by physiological changes to the body
Sleep and circadian rhythms
A night's sleep follows predictable stages and specific sequence of events. Our sleep-wake cycle is an example of circadian rhythm and is controlled by our internal body clock (the suprachiasmatic nucleus). Our arousal, hormone secretions, metabolism, HR and temperature all follow a circadian cycle
Sleep and ultradian rhythms
Within a night's sleep, we shift between NREM sleep and REm sleep. One cycle of NREM and REM sleep last for about 90 minutes and adults get between 4-6 cycles per night. Time in REM sleep increases and NREM decreases as the night progresses
Non-rapid eye movement sleep (NREM)
Stage 3
Stage 4
Stage 2
Stage 1
When we move from being awake to being asleep, we enter a hypnogogic state where we may experience flashes of light or hallucinatory images. We may also experience hypnogogic (hypnic) jerks - involuntary muscle spasms. It lasts for 30 seconds to 10 minutes (5 minutes average) and we are easily awakened. Stage 1 is sometimes called pre-sleep
Is really when sleep begins. Our first stage 2 lasts for about 20 minutes. it is easy to be woken up during stage 2 sleep. Our eyes becomes still, our muscles relax and bodily arousal decreases. This stage accounts for about 50% of a night's sleep
A brief transition before stage 4 and marks the start of deep sleep. we are less responsive to external stimuli. If awakened, we are groggy and disorientated. Our eyes do not move and our muscles are deeply relaxed
This is the deepest stage of sleep and it is very hard to wake someone up in this stage. The first stage 4 of the night is about 30 minutes long. We then cycle back through the stages to stage 2. Later in the night, sleep cycles do not return to stage 4 sleep
Rapid eye movement sleep (REM)
During REM sleep, our eyes move quickly under our eyelids. The first REM sleep lasts 10 minutes, but subsequent ones last progressively longer. If woken from REM sleep, people usually report that they were dreaming. During REM sleep, bodily arousal increases, but muscle activity id very low - the muscles show cataplexy - a form of paralysis. REM sleep is sometimes called paradoxical sleep - the body shows virtually no muscle activity but the brain and other systems are highly active. Most dreaming occurs in REM sleep and dreams get longer as the night progresses. Anxious people and light-sleepers also show dreams in NREM sleep
EEG recordings during sleep
1: Alpha and theta; 2: Theta, sleep spindles, K-complex; 3: theta and delta; 4: delta