Sunlight and Mood

Biological Clock

Information about light levels passes from the retina of the eyes to the suprachiasmatic nucleus (SCN) of the hypothalamus, deep inside the brain. The SCN controls the body’s circadian (daily) rhythm, including sleep–wake pattern and body temperature cycle. The SCN uses the light-dark information from the eyes to synchronize its own free-running biological clock (with a cycle time of just over 24 hours) to the external day–night cycle. In animals the same system also governs seasonal cycles, such as hibernation or colour changes.

One response of the SCN to light level is to control the production of melatonin by the pineal gland, also located deep inside the brain. Melatonin production is suppressed while light is present, and permitted while it is absent (or at a low level). Melatonin is a hormone that promotes sleep and acts as a mild tranquilizer.

Melatonin is produced from serotonin, a very similar chemical but with very different effects. Serotonin plays an important role in the control of mood, including anger, aggression, and depression, and also of body temperature, sexuality and appetite. High levels of serotonin promote alertness and activity, and generally make you ‘feel good’. Since melatonin is produced at the expense of serotonin, serotonin levels fall as melatonin production increases (at night), and recover again when melatonin production ceases (during the day).

Therefore, to keep your daily biological clock and associated cycles healthy, you need a good period of strong light during the day (to properly suppress melatonin production), and good period of true darkness during the night (to allow good melatonin production), together with a period of low light level prior to going to sleep.

Disruption to this daily pattern of light and dark, such as occurs with travel between time zones or temporary shift work, or inadequate differentiation in light levels between night and day, such as for night shift workers with inadequate light levels at work, exposure to bright sunlight on the way to and from work, or inadequate darkness when trying to sleep, thus leads to weak or mistimed variation in melatonin and serotonin levels. This in turn results in tiredness, lack of energy, poor concentration and alertness while awake, and difficulty in sleeping properly.

There is also evidence that melatonin slows the growth of cancerous tumours, including those for breast, prostate, skin and colon cancers. As a result, high light levels at night, such as those experienced by night-shift workers can increase the risk of cancer: women night-shift workers, for example, have a 60% higher risk of breast cancer.

Seasonal Affective Disorder (SAD)

Sufferers of this condition experience serious mood changes, often similar to depression, during the winter months, and normal mental health throughout most of the rest of the year. It is therefore also known as winter depression. Typical symptoms include sleeping too much, lack of energy and listlessness, and cravings for sweet and starchy foods. In extreme cases the depression can be severe, requiring hospitalisation or even leading to suicidal tendencies.

SAD is particularly common in the Nordic countries, and in Ireland, with about 1 in 5 people affected to some extent. The incidence in the northern USA is also high. It is also more common in women than men, by about 3 to 1. SAD can also occur in people who work indoors, especially those who work away from windows. These people also often suffer the symptoms for most if not all of the year

SAD is believed to be due the lack of sunlight during the winter months at high latitudes, where the days are short and the Sun does not rise very high in the sky. This is probably due to a combination of a lack of serotonin (causing depression) and an excess of melatonin (causing sleepiness), due to weakened synchronization of the body’s natural clock with the normal periods of wakefulness and sleep.

The main treatment for SAD is regular exposure to bright light; ideally natural sunlight but very bright (many times brighter the normal room lighting) artificial lights are often used instead.

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