Shakespeare tells us that sleep is a gentle thing, “nature’s soft nurse” and Keats describes it as a magical thing that “like a comfortable bird broods over the troubled sea of the mind till it is hushed and smooth”. Scientists view it rather differently…

Like clockwork

Modern day scientists tell us that sleep is not a passive event but a dynamic, active process controlled by the body’s internal, biological (circadian) clock located in the hypothalamus (in the brain). This nifty clock times and controls our sleep/wake cycles and functions according to a natural, built-in rhythm of just over 24 hours. It is, however, forced to adapt to and reset its own internal cycles to match the day length of the external environment. This process is triggered by light signals that are sent to the brain via special types of cells in the retina of the eye.

When our biological clocks are unable to make rapid adjustments to changes in light cues, such as travelling across time zones, jet lag occurs and it takes the body a number of days to adjust to the new time zone and light cues. Seasonal changes such as shorter days (less sunlight) and longer nights during winter may cause seasonal affective disorder (SAD), a form of depression that is often treated with light therapy.

Other rhythms controlled by our biological clocks that are related to the sleep cycle include body temperature changes and the release of melatonin and other hormones, for example growth hormones, while we sleep.

Sleep cycles and sleep states

Scientists have also identified the following two basic states/stages of sleep.

    1. Non-REM (non-rapid eye movement) sleep
    1. REM (rapid eye movement) sleep.

We experience repeated cycles of non-REM and REM sleep during the night. We usually nod off into a stage one non-REM sleep (a very light sleep) during the first part of the night and progress to a stage four non-REM sleep which is a deep sleep that is seldom affected by external stimuli or noise. During this cycle muscle activity may be low but muscle tone is normal and the body is generally in a similar state to being awake. This cycle usually lasts about 90 to110 minutes and is repeated four to six times per night until the REM sleep cycle takes over.

The REM sleep cycle is characterised by rapid eye movements (without opening the eyes) and almost complete paralysis of the muscles that move our bodies. However, during the REM sleep cycle, the heart, diaphragm, eye, intestine and blood vessel muscles continue to function and certain parts of the brain remain as active as when we are awake. This cycle is also thought to be one of the triggers that may cause us to dream. Fortunately, the muscle paralysis associated with this cycle prevents us from acting out potentially violent behaviours associated with our dreams.

How much is enough?

Although individual sleep needs vary, adults seem to need at least eight hours of sleep per night, teenagers at least nine or more hours per night, toddlers (3 to 5 years old)10 to12 hours and infants 16 to18 hours per night.

Sleep has healing powers

Sleep should be a natural part of every person’s life. A good night’s sleep is essential to your health and wellbeing and has many benefits both mental and physical. It’s a stress reliever that reduces activity in the brain regions that control decision making, emotions and social interaction and helps us conserve energy and other resources that our immune system needs to fight infections. However, it also helps us retain what we have learned and memorised and is essential when studying or writing exams. Even John Steinbeck concluded that “a problem difficult at night is resolved in the morning after the committee of sleep has worked on it”.

 

Sources
Sleep, sleep disorders and biological rhythms. 2003. Retrieved from:  http://science.education.nih.gov/supplements/nih3/sleep/guide/info-sleep
Sleep quotes. Retrieved from: http://www.brainyquote.com/quotes/quotes