Artificial light has transformed how the human race has lived in the past 100 years or so. It's estimated that between 1950 and 2000, exposure to artificial light in the UK rose four-fold. Even for those not working night shifts, it can be almost impossible to avoid the night-time glare from street lamps, car lights, TVs, computers and smartphones.
Now there are growing concerns that constant exposure to light when it's dark damages our health by disrupting our circadian rhythm - the body's built-in clock. Artificial light affects the production of melatonin, a hormone released by the pineal gland in the brain.
Melatonin triggers feelings of sleepiness and is released when it gets dark, with levels peaking at around 2am.
During the day, exposure to sunlight stimulates a nerve pathway from the retina (the light-sensitive area at the back of the eye) to an area in the brain called the hypothalamus, which is the control centre for the body clock.
The incoming signals help control hormones, body temperature and other functions that play a role in making us feel sleepy or wide awake.
Once this area receives a signal from the eye that the day has broken, it halts the release of melatonin from the pineal gland. When the sun goes down and darkness occurs, it tells the pineal gland to start releasing the hormone again.
As well as triggering sleepiness, melatonin has other crucial roles, including keeping blood pressure and blood sugar levels healthy.
'Bright light at bedtime suppresses melatonin production, so is probably going to delay sleep and affect the body clock,' says Professor Jim Horne, former head of sleep research at Loughborough University. 'But some people are more sensitive to it than others.'
Repeatedly delayed or broken sleep has been found to raise the risk of heart attacks by up to 50 per cent and strokes by 15 per cent. So how could artificial light be harming your health without you realising?
Piles on the pounds
Scientists at Northwestern University in Chicago recently found exposure to light from computers or smartphones at night could lead to weight gain. Even charging a phone by your bedside could have an effect.
They discovered exposure to such light increases hunger pangs that can last several hours, even if a meal was eaten that evening.
One reason may be that laptops and phones tend to emit more blue light, the kind that usually acts as a wake-up call for the brain and tells our body clock it's time to get up.
Exposure to blue light - the kind from smartphones, tablets and laptops - could leave you feeling drowsy
The beam produced by ordinary light bulbs does contain blue light but it's less concentrated than in some modern devices.
The study found being exposed to blue-enriched light (the kind smartphones produce) for three hours in the evening stimulated appetite, possibly by tricking the body clock into thinking it was the start of the day and time to replenish its energy stores with food.
This is because melatonin also helps to suppress levels of ghrelin, a hormone that increases appetite, while stimulating the release of leptin, a hormone that reduces food cravings.
'A small amount of blue light is not likely to have much effect on sleep or health,' says Professor Horne. 'But the brain is more sensitive to blue light than any other colour and it does tend to affect the body clock more than other light colour.'
May increase the risk of cancer
Occasional use of devices late at night is unlikely to cause any harm. But repeated exposure has been linked with an increased risk of cancer.
A recent report in the European Journal of Cancer Prevention featured findings from scientists who analysed 16 studies on light exposure at night.
The researchers, from a Chinese university, found that repeated light exposure - most of the studies involved night workers - increased the risk of breast cancer in women by 17 per cent.
But only 'high' artificial light, such as the glare from phones or laptops, semed to have this effect. Ambient lighting which illuminated a wider area, such as a bedside lamp, did not appear to significantly increase the risk.
Other studies found nurses who work nights are at a higher risk of the disease and that light exposure makes some tumours resistant to one of the main weapons against the disease - breast cancer drug tamoxifen.
It's not clear why artificial light at night might have this effect - one theory is that by disrupting the natural hormone balance in the body it creates the right environment for cancer cells to flourish.
Another suggestion is that reduced melatonin output at night allows an increase in levels of another hormone, oestrogen, which is involved in the development of two in every three cases of breast cancer.
Recent studies found female hamsters repeatedly exposed to fairly low levels of light at night showed signs of low mood and depression
Makes children more anxious
Animal research suggests night-time exposure to artificial light could make children more anxious.
A study in the journal Physiology and Behaviour found baby mice exposed to light at night from a young age displayed more signs of stress and anxious behaviour than those sleeping in dark conditions - possibly by increasing levels of stress hormones such as cortisol.
Another study, published last month, found children may be more susceptible to melatonin suppression from night light than adults.
Scientists at Kyushu University in Japan found that while 46 per cent of grown-ups exposed to light at night saw a significant drop in melatonin production, in children the figure was 88 per cent.
Researchers said: 'The percentage of melatonin suppression by light in children was almost twice that in adults.'
Artificial light from charging smart phones or tablets by the bed may not be the only problem. Some studies suggest lights in other parts of the house are a risk.
Research in the journal Chronobiology International found elderly people were much more likely to develop diabetes if they sat under bright lights for four hours before they went to bed, rather than dimmer ones.
Japanese scientists looked at 513 men and women in their 70s and found the brighter the bulbs, the lower the melatonin level.
When they examined type 2 diabetes rates they found those exposed to brightest lights in the evening were 50 per cent more likely to develop the disease. This may be because the body clock also has a role in controlling blood sugar levels.
One solution, says Professor Horne, may be to switch to red light bulbs. 'The body clock is not so sensitive to red light so if you want to avoid adverse effects of light at night then it is an option.'
Could give you the blues
A restful night's sleep is meant to do the power of good. But exposure to night-time light could have the opposite effect on mental as well as physical health.
Recent studies at Ohio State University found female hamsters repeatedly exposed to fairly low levels of light at night showed signs of low mood and depression.
Scientists found hamsters exposed to dim light had higher blood levels of tumour necrosis factor (TNF), a protein that usually sends messages in response to injury or inflammation.
Constant exposure to raised TNF levels has also been linked with depression because it tends to be picked up by receptors in the hippocampus, an area of the brain that not only regulates the body clock but that is also involved in regulating mood.
Drowsiness in the daytime
Exposure to blue light - the kind from smartphones, tablets and laptops - could leave you feeling drowsy the next day, even if you get a good night's sleep.
That's the conclusion of a recent study at Tsukuba University in Japan, where nine men were exposed to either blue light or no light for two hours before bed. Although they slept for similar times, the next morning researchers found higher levels of drowsiness among those shown the blue light.
Their metabolisms were slower, suggesting light had disrupted the body clock's ability to kick-start things in the morning. This could be linked to a fall in melatonin levels in the night.
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