You may eat healthily and exercise regularly, but you should add turning out the lights to your list of healthy habits, according to a new study.
Research has shown that being exposed to constant levels of light by having lamps on at night, can have some negative negative health effects.
Based on an experiment in which mice were kept under conditions of constant light for months, it warns constant exposure to light can lead to frailty.
'Our study shows that the environmental light-dark cycle is important for health,' said Johanna Meijer of Leiden University Medical Centre in the Netherlands.
'We showed that the absence of environmental rhythms leads to severe disruption of a wide variety of health parameters.'
Those factors included the inflammatory response by the immune system, muscle loss, and early signs of osteoporosis. The researchers said the physiological changes they observed were all indicative of 'frailty' as is typically seen in people or animals as they age.
However, the study, published in the journal Current Biology includes some encouraging news.
Dr Meijer said: 'The good news is that we subsequently showed that these negative effects on health are reversible when the environmental light-dark cycle is restored.'
To investigate the relationship between a loss of the light-dark cycle and disease, Dr Meijer her and colleagues exposed mice to light around the clock for 24 weeks and measured several major signs of health. Readings showing the animals' brain activity revealed constant light exposure reduced the normal rhythmic patterns in the brain's central circadian pacemaker - the suprachiasmatic nuclei - by 70%. The disruption to normal light and dark patterns and the circadian rhythm also led to a reduction in the animals' skeletal muscle function when the researchers measured their strength.
Worryingly, their bones showed signs of deterioration, and the animals entered a pro-inflammatory state normally observed only in the presence of pathogens or other harmful triggers of disease. But after the mice were returned to a standard light-dark cycle for two weeks, the brain cells rapidly recovered their normal rhythm, and the animals' health problems were reversed.
The findings suggest more care should be taken in considering the amount of light exposure people get, particularly those who are ageing or vulnerable.
It's estimated 75% of the world's population is exposed to light during the night and 24-hour lighting is common in nursing homes and intensive care units, with patients, doctors and nurses exposed to it around the clock.
'We used to think of light and darkness as harmless or neutral stimuli with respect to health,' Dr Meijer said.
'We now realise this is not the case based on accumulating studies from laboratories all over the world, all pointing in the same direction.
'Possibly this is not surprising as life evolved under the constant pressure of the light-dark cycle.
'We seem to be optimized to live under these cycles, and the other side of the coin is that we are now affected by a lack of such cycles.'
While the researchers are certain that 'light exposure matters,' they are planning more in-depth analysis of the influence of distorted light-dark cycles on the immune system and would like to investigate possible health benefits to patients exposed to more normal conditions of light and dark.