Ancestral sleep

We evolved on a rotating planet, with regular patterns of light and dark exposure, and our biology is set up to work with this cycle. Our kidneys produce less urine, our body temperature is lower and our immune systems are less capable of fighting off foreign invaders at night. During the day, blood pressure rises, hunger hormones kick in and our brains shift into a higher gear. These daily fluctuations in our biology are called circadian rhythms, and they dictate when we feel sleepy.

For most of human history, we slept at night and were more active during the day. Some historic accounts (Roger Ekirch) suggest that our ancestors split the night into two sleep shifts, with a period of restful wakefulness in between. However, this may not have been a universal human trait: studies of traditional societies, such as tribes in Africa and South America, and the Old Order Amish in North America (who live without electricity) suggest that many of these societies sleep in a single chunk – like most people in the modern world.

These same studies have found that, on average, people from traditional societies go to bed and wake up several hours earlier than we do in developed countries. Not only do they sleep earlier than us, they also seem to sleep better. Between 10 and 30% of people in developed countries experience chronic insomnia, whereas just 1.5% of Hadza people (Tanzania), and 2.5% of San people (Namibia) say they regularly have problems falling or staying asleep. Neither group has a word for “insomnia” in their language.

Lightbulb moment

In 1879, Thomas Edison flicked a light switch, and our relationship with night and day changed for ever. His incandescent lightbulbs brought cheap electric lighting to the masses, enabling us to extend the day. But this also changed the way we sleep. Exposure to light at night makes us more owlish, so we feel sleepy later. Light also has a direct alerting effect on the brain, meaning we find it harder to sleep.

Shift work

The transition to gas and then electric lighting also made shift work easier: no longer did factory workers have to toil under weak candlelight. Working at night means being awake, active and eating during the biological night – and then trying to sleep when the body is in day-mode. People who work night shifts are estimated to lose between one and four hours of sleep per day, which has short-term effects on emotional stability, memory, reasoning, reaction speeds and hand–eye coordination. Chronic sleep deprivation also precedes the onset of Alzheimer’s disease, cancer and various psychiatric illnesses, and is also associated with heart disease, obesity and diabetes. Similar links have been made to disrupted circadian rhythms, another consequence of shift work. If you are eating and active during the biological night, the circadian clocks in your various organs and tissues become desynchronised, meaning they no longer work as efficiently.

Daylight saving time

British Summer Time was introduced in 1916. The hope was that the extended evening light would encourage participation in outdoor recreation, facilitate military training and reduce industrial energy consumption. There is a significant downside, however: by moving the clocks forwards each spring, and backwards each autumn, we force a change in the timing of our body clocks. In the US, which also seasonally adjusts its time-keeping, a study of high school students – a population that’s already sleep-deprived – suggested their sleep was curtailed by 32 minutes per night in the week following the spring clock change; their reaction speeds and vigilance also temporarily reduced. In adults, the transition to summer time has been associated with a 6% increase in “cyberloafing” – spending one’s work time on non-work-related websites – on the Monday after the clock change. From a health perspective, the clock changes are tied to an increased risk of heart attacks, strokes, suicide attempts and psychiatric admissions.

Jet lag

American travel writer Horace Sutton is credited with the first reference to jet lag, in the Los Angeles Times in 1966. The most obvious symptom of the condition is feeling tired at the wrong time of day, but jet lag can also trigger mood changes and constipation. This is because besides the circadian clock that governs sleep timing, there are also clocks in our brain cells and digestive organs, which must also adjust.

To overcome jet lag, we’re often told to start living on the time zone of our destination as soon as we arrive, but this can be detrimental. Say you’re travelling from New York to London – which requires your body clock to advance by five hours. If you take an overnight flight and arrive at Heathrow at 8am, it will be 3am according to your body clock – and exposure to daylight at this time is the opposite of what you want to achieve. In this situation, it would be advisable to wear sunglasses until about 11am (6am, according to your body clock).

More than meets the eye

It is only in the past two decades that the biological mechanism by which light affects the timing of our internal clocks has been explained. We now know that at the back of the eye there is a subset of light-sensitive cells, which connect to a small patch of brain tissue called the suprachiasmatic nucleus (SCN). They also link to areas of the brain that regulate mood and alertness. The SCN is the biological equivalent of the Greenwich meridian: it is the reference point used by the billions of other cellular clocks in your body to remain accurate. When light penetrates the eye, it acts like the reset button on a stopwatch: it tweaks the precise timing of the SCN, ensuring it remains aligned with the rising and setting of the sun.

Digital revolution

The spread of tablets and smartphones has undermined our relationship with the night still further. Light-sensitive cells in the back of the eye are particularly sensitive to blue wavelengths of light, which predominate in electronic screens. Standard LED bulbs also emit large amounts of blue light, even though they appear bright white. Recognising the impact that this light might be having on our sleep, the American Medical Association recently issued guidance on LED streetlights. They are being rolled out because they are more energy efficient than older lights – but their effect on people’s circadian system is also five times greater. The AMA advises communities to avoid standard blue-white LED streetlights and opt for warmer coloured ones – these should also be dimmable. It’s advice we should also adopt in our homes if we want to improve our sleep: once the sun sets, keep lighting warm and dim. You should also install an app or alter the settings on smartphones and computer screens to dim and filter out blue light after dusk.

Lighting the future

Many different-coloured LEDs can be joined together to vary the shade of the light they produce, enabling the colour and intensity of a lighting system to be adjusted according to the time of day. Already, some workplaces, hospitals, and care homes are installing such “circadian” or “human-centric” lighting systems, to try to boost daytime productivity and improve people’s sleep, with promising results. The biggest challenge is getting more light into people’s eyeballs during the daytime. But there’s a simple solution: even on an overcast winter’s day, it is at least 10 times brighter outside than in a standard office – by walking to work and getting outdoors at lunchtime, we can begin to reconnect with the light cycle we evolved under. There’s even evidence that daylight exposure will improve your sleep.

• Linda Geddes is the author of Chasing the Sun: the New Science of Sunlight and How It Shapes Our Bodies and Minds, published by Profile/Wellcome