Sleep is a form of altered state of consciousness.
Sleep is, ultimately, needed for survival. Animals – including humans – treat sleep in different ways. Some animals, for example, sleep only during the day, where they would otherwise be most vulnerable to predators. Other theories of why sleep is required for survival include the following:
There are five sleep stages, each lasting roughly ninety minutes (although stage 1 sleep tends to be shorter):
As you can see, the first four stages of the sleep cycle encompass non-rapid eye movement sleep, whilst the last stage (stage 5) is rapid eye movement sleep. What is the difference between NREM and REM?
There are four stages of NREM sleep (the first four stages in a regular sleep cycle), which account for roughly 75-80% of our total sleep time. During NREM sleep, brain waves become increasingly regular, slow, and large. Each stage is characterised by a) different types of brain waves, and b) different types of activity, as we will see below.
For comparison, it should be stated that when we are awake and alert (in NWC), we have beta brain waves. Beta brain waves are high frequency and low amplitude.
Stage 1 NREM
Stage 1 sleep is often so light that you may think you haven’t slept at all. When you are very relaxed and about to fall asleep, you will have alpha brain waves: fairly high frequency, fairly low amplitude. As stage 1 sleep develops, these will turn into theta brain waves: medium frequency, with mixed amplitude. In stage 1 sleep, you may experience a hypnic jerk, whereby you suddenly feel like you have just landed from a significant fall.
Stage 2 NREM
Stage 2 sleep is also characterised by theta brain waves, but includes the phenomena of sleep spindles and k-complexes. Sleep spindles are sudden, short bursts of rapid brainwave activity, whereas k-complexes are single bursts of a high-amplitude wave. These phenomena are illustrated below.
Stage 3 NREM
In stage 3, our sleep becomes much ‘deeper.’ In the transition from stage 2, stage 3 begins with theta brain waves, but these quickly give way to delta brain waves: low frequency, high amplitude. In ‘deep sleep,’ an individual may be more difficult to wake. Experiencing deep sleep is crucial in feeling rested the next day.
Stage 4 NREM
Stage 4, like stage 3, is characterised by delta brain waves. In stage 4, these low frequency, high altitude waves are even more prevalent. In both stage 3 and stage 4 NREM sleep, a sleeper may experience night terrors, sleepwalking and/or bed-wetting.
REM sleep, as the name suggests, is characterised by rapid movement of the eyes during sleep. It accounts for roughly 20-25% of our total sleep time, however this figure is as high as 50% for infants, and may change slightly from person to person. Already exemplified by rapid eye movement, brain waves during REM sleep suggest that our mind is active: as opposed to brain waves typical in NREM sleep, REM sees irregular, high frequency, low amplitude waves.
As the brain is active but the body almost paralysed during this stage, REM sleep is sometimes referred to as ‘paradoxical sleep’. It is during this stage that most dreaming (especially vivid dreaming) occurs, and one’s heart rate, blood pressure and respiration is likely to increase.
Stage 5 (REM sleep)
Typically, we experience 5-6 cycles of REM sleep per night. Brain waves during REM sleep are beta-like, which further suggests that the mind is active, and includes sawtooth waves, which are theta-like.
Below is a simplified version of a regular sleep cycle. The portions in red at the end of each sleep cycle (in line with stage 1) indicate REM sleep.
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