We all need sleep, but few of us ever consider what it is for. One thing is certain however, we know when it seems not to be right, because poor sleep has a major impact on life.
Sleep is essential for health and wellbeing, but requirements are different for different mammalian species. For instance, the Giraffe only sleeps for about 10% of its day, Humans for 30%, Cats for 50%, and the Little Brown Mouse in North America for an amazing 95%! Furthermore, some animals hibernate for many months at a time – so we still have a lot to learn about the reasons for sleep. If we live for 70 years, over 20 years will have been spent asleep!
Sleep is divided into two, Dreaming, Rapid Eye Movement (REM) sleep, and non dreaming known as non-REM sleep. Non REM sleep is further subdivided into three stages (one to three) with stage three (now combined with the old stage four) sleep being the deepest. When we sleep, we cycle through these different types of sleep in an ordered and well defined manner – it like a jigsaw that needs to be put together correctly. We call it Sleep Architecture. Any disturbance to the architecture, for instance frequent awakenings from snoring, or disturbances to the timing or depth of sleep, will have consequences for refreshment and feeling good the following day.
There is only one way to find out definitely what type of sleep you are in at any one time, and that involves measuring the electrical activity of the brain by electroencephalography (EEG). Such monitoring of the brainwaves during sleep can be used to produce a hypnogram (see image below).
This is complicated, and cannot be done reliably by simply placing a smart phone under the pillow, despite the claims of some smart phone apps! Combining this with the monitoring of other physiological parameters (respiratory, cardiovascular and muscle activity) is called polysomnography (PSG). This is the gold standard for diagnosing sleep disorders, but requires specialist care.
Sleep architecture refers to the different depths and planes of sleep that are important to gain refreshment and to feel well rested after waking.
- 0–20 minutes: After lights out, the phase before dropping off to sleep is called the sleep latency, and is usually about 15–20 minutes long.
- 20–90 minutes: After sleep onset, non-REM sleep begins with a rapid decent through sleep Stages 1, 2 and 3 into Stage 4 deep Delta Wave/Slow Wave Sleep.
- 90–120 minutes: First episode of REM sleep lasting 30–45 minutes.
- 120–180 minutes: Second episode of non-REM deep slow wave sleep.
- 180–end of sleep: Alternating cycles of REM and lighter Stage 2 non-REM with increasing predominance of REM later in the night.
Sleep architecture is usually displayed graphically in a Hypnogram (see image above).
During REM sleep we dream, and our brain waves look similar to those we emit when we are awake, because of the experiences and thought processes of dreaming.
REM sleep normally begins about 90 minutes after sleep onset, and follows a period of non-REM sleep. As the night progresses, phases of non-REM and REM sleep alternate, and by the end of the night three or four phases of REM sleep have occurred, mainly in the second half of the night. If we wake during REM sleep, which is more likely at the end of the night, we will remember dreams, and remembering dreams is very common after sleep is disrupted, for example by a visit to the bathroom.
REM sleep is so called because the eyeballs move around rapidly during this phase of sleep, and this can be seen clearly by looking at the eyelids of a person who is dreaming. During REM sleep, the muscles of the body are totally relaxed (except for the muscles of breathing). This stops us from acting out the dream that we are having at the time. Sometimes this relaxation process does not work properly, and dream enactment does occur. This is called REM Behaviour Disorder (REMBD).
Relaxation of the muscles during REM sleep may lead to two other important effects.
Firstly, the throat muscles also relax causing the throat to be at its narrowest in REM sleep. This often makes snoring and breathing worse when we dream.
Secondly, the muscle relaxation occasionally continues after we wake, giving a feeling that we are unable to move. Although this is quite common and is harmless, it can be distressing. Most importantly however, it also occurs most often in people suffering from Narcolepsy.
Non-REM sleep is subdivided into four stages of depth, which are identifiable by recording and analysing the brain waves.
Stage 1 sleep is the lightest and Stage 4 (now combined with Stage 3) is the deepest, and is called Delta Wave/Slow Wave Sleep, because of the character of the brain waves, which have low frequency and high amplitude.
Physiologically, the body is dormant and relaxed with a fall in blood pressure and heart rate, and slow quiet breathing. It can be difficult to wake people in slow wave sleep, and arousal may be associated with confusion and mis-interpretation of the environment.
Normal sleep starts with a period of Non-REM for the first 90 minutes after sleep begins (sleep onset). This phase of sleep is the most important for refreshment the following day, with adequate amounts of slow wave sleep being the essential element. Disturbances to this phase of sleep by Snoring and Obstructive Sleep Apnoea or Periodic Limb Movements during Sleep (PLMS) are the most common causes of morning lack of refreshment and excessive daytime sleepiness.
REMBD occurs when the normal relaxation of the muscles during REM sleep does not switch on, and the sufferer acts out their dreams. Characteristically, the dreams are unpleasant or violent, and pushing, shouting, punching or kicking are often observed by the sleep partner. Sufferers often dream about being attacked and having to defend themselves. There may be a history of falling out of bed in the night.
Because the condition is associated with REM Sleep, it is more likely to occur later in the night rather than earlier, and this can distinguish it from non-REM parasomnia, confusional arousal, sleep walking or sleep talking, which occur earlier in the night.
Sufferers of REMBD are usually male and in late middle age or elderly age groups.
It has been suggested that this is a neuro-degenerative disorder, possibly associated with a future risk of developing Parkinsons Disease. Suffers should seek medical advice from a sleep physician.