4. What effects on health have been observed?
- 4.1 Thermal and chemical effects.
- 4.2 Effects on the eyes
- 4.3 Effects on the sleep, mood and the circacian rhythm.
4.1 Thermal and chemical effects.
Exposure to light at night can disrupt the circadian rhythm
The body has a pain reflex that makes people move away when they feel a burning sensation so for a light source to cause burns, it must be very intense. Lasers or high-power flash lamps fall in this category but these are not normally used for illumination. If parts of the skin are regularly heated, this can lead over a long period of time to “erythema ab igne”, which is associated with skin cancer. However, this is very unlikely from light sources used for illumination.
The skin can adapt to gradually increasing levels of UV exposure and in some people this causes tanning. However, it is harder for the skin to respond to sudden changes in UV as when people living in northern latitudes take a winter holiday in a very sunny resort. Some photosensitive individuals also find that their symptoms become worse in spring or summer if they have not had the chance to acclimatize their skin to sunlight during the winter months.
Overexposure to UVA and UVB causes sunburn. At first the skin reddens and if the dose is high enough there is an inflammatory reaction causing an increase in temperature and swelling, and after a few days the skin peels. Overexposure to UV or long term exposure to doses of UV which are just below those needed to cause sunburn, can aggravate bacterial and viral infections and lead to skin cancer. People who have been sunburnt severely many times, especially in childhood, are more likely to develop melanoma, the most fatal of skin cancers, as well as squamous cell carcinomas. Over the last few decades there has been an increase in the incidence of skin cancers, probably because of an increase in exposure to sunlight during holidays and leisure time.
Most of the lamps used for lighting are considered safe but some emit UV radiation. Under extreme circumstances and over a long period of time, exposure to these lamps could increase the chance of developing skin cancer in later life. The added personal risk is very small but there could be a significant (in the hundreds of cases) number of cancer cases over the whole population. This added risk could be virtually eliminated if the light was covered with plastic or glass that absorbs UV. More...
4.2 Effects on the eyes
UV and IR radiation cause lesions in the cornea and the lens of the eye but these can often be repaired. Long term exposure to UV from sunlight can also cause cataracts of the lens. However, these types of damage are very unlikely from either short-term or long-term exposure to lamps used for lighting.
Damage to the retina is usually untreatable and permanent. Normal artificial lights are not intense enough to cause burns but they can initiate chemical reactions that proceed to harmful levels. The retina is particularly vulnerable to short wavelength radiation such as blue light and the susceptibility increases with age.
Natural pigments and vitamins in the eye protect the retina by mopping up free radicals and reactive species but, over time, people can develop age-related macular degeneration (AMD), particularly if they smoke. There is no evidence that sunlight exposure early in life may contribute to retinal damage that would lead to AMD in later life, but exposure to blue light could, particularly for older people. More...
4.3 Effects on the sleep, mood and the circacian rhythm.
Life on Earth has evolved around a 24-hour day with roughly 12 hours of daylight followed by 12 hours of dark; and many biological processes follow this circadian rhythm. In mammals, this 24-hour “clock” is controlled by the hypothalamus but is also affected by external factors, mainly light.
Specific photoreceptors in the retina receive information on light and send the signal directly to the hypothalamus and to other parts of the body that influence sleep, mood and memory.
The production of melatonin, a powerful hormone, is also ruled by the circadian rhythm so that it is synthesized almost exclusively at night. This hormone sends signals to the rest of the body to tell whether it is day or night and promotes sleep. Melatonin also has other important roles as an antioxidant and a protective agent against “wear and tear” in tissues.
Exposure to light in late evening, at night or early morning disrupts the circadian rhythm and the production of melatonin, and hence has an effect on sleep, mood and cognition. Severe disruption of circadian rhythms is linked to breast cancer and could also play an important role in the development of breast, prostate, endometrial, ovary, colorectal and skin cancers; cardiovascular diseases, reproduction, endometriosis, gastrointestinal and digestive problems, diabetes, obesity, depression and sleep deprivation.
Light itself has an effect on alertness, sleep, mood and the circadian rhythms regardless of the type of lamp used. Blue light or light enriched in blue has a more pronounced effect than other colours or white light but lamps available to the general population are not blue or blue-enriched. More...