Sleep alternates between two phases: orthodox sleep and REM sleep. These phases can be distinguished from one another in EEG (electroencepha- lography). The majority of sleep is orthodox sleep (deep sleep, quiet sleep, slow-wave sleep) that can be further divided into three NREM (non-rapid eye movement) stages: N1, N2 and N3. These are in contrast to REM sleep, or R sleep (paradoxical sleep, rapid eye movement sleep).
W – Wakefulness (beta waves): infrequent and low- frequency beta waves predominate in the EEG.
Meditative state with one’s eyes closed: increasingly synchronised alpha and theta waves are visible in EEG, along with Increased production of serotonin. A number of proven health bene ts have been observed while using techniques such as meditation to increase one’s alpha and theta waves.
N1 – The first stage (theta waves, 4–8 Hz): EEG shows irregular oscillations. Theta waves are slower and higher in frequency than alpha waves. This is a transitory phase from wakefulness to light sleep. The sleeper changes position frequently, and is in a deep meditative state.
However, if someone were to wake the person up, he or she might not feel like they had fallen asleep. Duration: about 10 minutes.
N2 – The second stage (sleep spindles, 11–16 Hz): A period of light sleep, during which there is little movement and the breathing is quiet. The second stage involves periodic surges in brain wave frequency, the so-called sleep spindles. Brain activity during the second stage is more active than in the first stage. Dreaming becomes possible. Getting enough stage two sleep improves motor skills. The person can still be easily woken up during this stage. Duration: 20 to 30 minutes.
N3 – The third stage (delta waves, 0–8 Hz): A period of deep sleep, where breathing is stable and EEG readings consist of slow delta waves. Muscles are completely relaxed, and the pulse, body temperature and blood pressure have decreased. Production of human growth hormone begins, and the regenerative mechanisms of the body are activated. The sleeper will not wake if another person walks into the room. Pulse, blood pressure and body temperature are at their lowest. Duration: 30 to 40 minutes. Elderly people experience a shorter duration, by as much as six minutes.
R – REM Sleep (alpha and beta waves): During REM sleep, the brain is awake, but the rest of the body is asleep. The muscles in the neck and the body are paralysed to prevent sleepwalking. During REM, the eyes are moving under the eyelids, and dreaming is at its peak. The typical adult has an average of 4 to 5 REM stages every night. The first stage lasts about 10 minutes, while subsequent stages are often longer, around 30 minutes. REM sleep is important for the regeneration of the brain’s nerve cells. Tests measuring the effects of sleep deprivation have shown that REM sleep is absolutely indispensable as deprivation leads to irritability, fatigue, memory loss and reduced capacity for concentration. Infants experience a lot of REM sleep: On average 50 % of the total 16 hours of sleep per night is REM sleep.
During a typical adult’s 7 to 8 hour sleep, the sleeper moves from the first stage, to the second, and to the third stage, then back again to the second stage. After this, the sleeper either wakes up or goes straight to REM sleep. From then on, the cycle repeats itself some 4–5 times.
DIFFERENT STAGES OF A SLEEP CYCLE AS SEEN ON EEG
One full cycle lasts about 90 minutes. From the perspective of getting a good night’s sleep, it is paramount to maximise the amount of deep sleep (N3) by going through at least three cycles. Getting enough sleep reorganises one’s memory and improves one’s learning capacity. In the later cycles, the amount of REM increases and the amount of deep delta sleep decreases, until eventually the latter disappears completely.
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