The Stages of the Sleep Cycle

Every night, when you to sleep, you transition through a number of stages, each of which is characterised by different types of brainwave, with a different effect on your sleeping body. Each cycle of sleep lasts 90 - 110 minutes. Let's take a look at the different stages in the sleep cycle. 

NREM Stage 1: alpha - theta waves (4 - 7 hz)

Stage 1 is a very light stage in the sleep cycle, which is it easy to be awoken from. Muscle atonia begins to set in, which causes twitches and hypnic jerks. Hypnagogic hallucinations occur, which often manifest as swirling, abstract colours which hypnotise you into sleep. You begin to lose self-awareness and consciousness and most sensory attachment to the external world as your brainwaves slow.

NREM Stage 2: mixed EEG activity

Stage 2 is marked by the brainwaves slowing even more, and a loss of virtually all muscle tone, so that your body cannot physically act out your dreams when you enter REM (Rapid Eye Movement) sleep. There are brief bursts of higher-level brain activity, known as 'sleep spindles' or K-complexes'. This is a light, dreamless stage of sleep, in which you spend almost half of your sleep time.

NREM Stages 3 & 4 - deep sleep: delta waves - below 4 hz

Stages 3 and 4 of the sleep cycle are known as deep, delta or slow-wave sleep, which is characterised by delta brainwave activity and a complete lack of consciousness. The sleeper is less responsive to external environmental stimuli and less likely to awaken as a result. This is a dreamless stage in the sleep cycle, when somnambulism (sleep-walking) is most likely to occur.

REM Sleep - paradoxical sleep: theta waves

REM sleep is characterised as paradoxical sleep - marked by higher levels of brainwave activity. This is when dreaming takes place. Gamma waves (30 hz and above) may be observed in lucid dreamers, this being a form of brainwave activity associated with higher perception and consciousness. 

REM sleep occurs throughout the sleep cycle. Here is an illustration of a typical 8 hour sleep cycle:

The longest, more intense and vivid dreams take place in the 4th and 5th periods of REM sleep, right at the end of the 8 hour sleep cycle, after approximately 6 hours of sleep. This means this is the optimal time for lucid dreaming to occur. The 4th an 5th stages of REM sleep can last between 45 - 60 minutes.

If you tend to wake naturally (without the use of an alarm clock), it is likely that you awaken directly from a dream, when it is fresh in your memory. It is possible to directly re-enter the dream if you allow yourself to immediately fall back asleep. 

The amount of sleep needed on average differs from person to person, and is also dependent on other variables such as age and general health. Newborn babies require the most sleep (14 - 17 hours per day), while adults tend to require approximately 7 - 9 hours of sleep on average. 

If you awaken with an alarm clock, you risk waking mid-cycle, which can be an unpleasant feeling - and you may miss the last periods of REM sleep, which are perfect for dreaming and more pleasant to wake up from. This problem can be overcome by understanding your sleep cycle, or employing the new technology and apps which can monitor your sleep cycle and wake you up once a cycle is complete.

EEG machines can be used to monitor how long a dream lasts. The EEG machine reads brainwave activity - and as you can see from the diagrams above, each stage in the sleep cycle has it's own distinctive brainwave activity associated with it. EEG brainwave monitoring indicates that dreams which occur in early stages of REM sleep last only a few minutes. As you then transition through different stages in the sleep cycle, including the deep, delta-wave sleep, it is unlikely that you will have a vivid memory of these early, fleeting dreams.The majority of the first sleep cycle is dedicated to non-REM sleep, due to the need to prioritise restful and restorative healing sleep.

Time may be experienced differently in dreams - there may be some time distortion - in particular time dilation, where the passage of time is experienced as being much longer within the dream. Generally, it's likely that dream time is relatively commensurate with real time, but it can be very difficult to judge the length of a dream from the first person perspective. Some people report that their normal, non-lucid dreams are experienced as stretching over days, weeks - even months or years. 

Just like normal, non-lucid dreams, lucid dreams can last from minutes up to the full period of REM sleep (i.e. 60 minutes). Lucid dreams may be cut prematurely short due to heightened arousal and the intense excitement of the experience waking the dreamer up. Lucid dream stabilisation can significantly help in prolonging the lucid dream experience as well as enhancing the clarity of the lucid dream.

There are a number of ways in which you can 'hack' the sleep cycle in order to maximise your chances of successfully experiencing a lucid dream - see The Dreamhacker Series | Hacking the Sleep Cycle (2) for a beginner-level tutorial on this topic.

Below, are some neuroimaging of the brain, showing different levels of mental activity and inactivity during REM sleep and comparisons to brain activity during wakefulness and NREM (delta, slow-wave) sleep.