The sleep-wake cycle is regulated by the body’s internal 24-hour clock – the circadian rhythm. But spending too much time indoors may be disrupting it.

Light is the strongest mechanism we have for synchronizing our internal body clocks with the world around us. But our everyday lives, which are often spent indoors in dim artificial light, easily throw our natural circadian rhythm out of sync.

And that isn’t only affecting our sleep. Researchers have shown as many as 73 traits – from alertness to strength – wax and wane depending on the time of day. Our mood also seems to be affected by the amount of light we are exposed to. Recent research shows that our ipRGCs – the cells that help regulate our internal sleep cycle – also connect to the thalamus, an area of the brain related to our mood.

While a one-off disruption to our sleep-wake cycle may typically leave us feeling less alert or sleepy during the day, a long-term breakdown of the circadian rhythm can lead to more serious health consequences. In all, dozens of diseases from cardiovascular dysfunction, to immune dysregulation and reproductive problems are all linked either directly or indirectly to circadian disruption. So how can you reset your internal body clock? Here are ways you can adjust the light in your life to help bring your natural sleep cycle back into sync.

Sunlight and your circadian rhythm

The circadian rhythm is an internal timing system that is thought to have originated in the earliest organisms on Earth, who developed it to protect their DNA from the Sun’s UV radiation. Light is still the most important mechanism that helps sync our internal body clock to the outside world. As the days get longer in the spring, for example, the light-sensitive ipRGC cells in our eyes detect and respond to this change.

But present-day lifestyles are disrupting our natural circadian rhythm by changing how much light we are exposed to. A key difference is the amount of time we now spend inside, which is around 90% of the day for many people. This can disturb our circadian rhythm because although indoor environments can seem pretty bright, artificial indoor lighting is often much weaker than we perceive it to be. While a typical classroom, office or hospital setting provides a light intensity that is between 150 and 1,000 lux, being outside on a sunny day can mean being exposed to light that is 10 to 1000 times brighter.

The difference in brightness between indoor and outdoor environments is not always obvious because the parts of our eyes that relate to vision are very good at adjusting to different light levels. Meanwhile, the circadian-regulating ipRGC cells in our eyes are highly sensitive to this difference.  

Luckily, one of the best remedies is also a simple one: daylight. A study from 2017, for example, showed that people who spent more time in circadian-effective morning light, took less time to fall asleep and had fewer sleep disturbances. To help reboot your circadian rhythm, try to spend a bit more time outdoors or close to a source of natural sunlight. For example, by eating breakfast either outdoors or in a sunny and bright part of your home. If you can, try to walk or cycle to work, take a lunchtime stroll or try sitting close to a window during the day.

Timing of light helps your body clock

For our circadian system, the timing of light exposure matters too. The best time to be outside in natural light, for example, is in the early parts of the day. That’s in part because ipRGC cells are especially sensitive to blue light, which is more prevalent during early daylight hours – the Sun’s rays shift to a redder light spectrum in the evening. Research has demonstrated that increased exposure to blue-enriched morning lighting improves both alertness and reaction speeds.

On the other hand, being exposed to bright light late in the evening suppresses the body’s production of the sleep-promoting hormone melatonin. So walking through a brightly lit grocery store in the evening, for example, may make it harder to fall asleep later.

And while getting plenty of light is important during the early parts of the day, darkness at night is essential to a good night’s sleep. Studies of people in areas with high levels of light pollution show that they tend to go to bed later and wake up later in the morning. The time they spend sleeping tends to be shorter and they report feeling more tired during their day.

A pre-sleep routine that involves gradually darkening your environment can help. Try dimming the lights and using table lamps instead of overhead lights. Turning off large screens such as your computer, TV or iPad can also help. Thick, full-length curtains, window shutters or even an eye mask can help create an even darker environment for better quality sleep.