You wake up at 6 am, which is close to sunrise but not quite close enough. Quick shower, coffee in hand, down to the underground parking, and off to the office. There you spend just enough hours to guarantee the sun has already clocked out by the time you drive home. Of course, you still have light. We aren’t living in the Stone Age, after all! Screens, lamps and fluorescent tubes keep you company all day long. But actual sunlight? Not so much.

For a week, it hardly seems like a big deal. Even a month might slip by unnoticed. Plenty of cities, with their grey skies and endless rain, treat the sun like an occasional guest anyway. But what if this routine stretched into years? What would happen to your body if you never saw daylight at all?

We tend to think of our eyes as tools for seeing. But they moonlight in a second, less obvious job: telling the brain what time it is.

Inside the retina sit special light-sensitive cells that play no part in recognising faces or reading signs. Their main job isn't to help us see the world, but to measure the light around us and send signals to the brain. These signals help regulate when we feel awake, when melatonin is released and when we start getting sleepy.

And here is something rather curious: our internal day isn’t exactly 24 hours long.

In a study of 157 people, researchers found that the average human body clock runs on a cycle of roughly 24 hours and 9 minutes. Nine minutes hardly sounds like much, but with no external signals to correct them, they can add up. In theory, after a month, your internal clock could be more than four hours out of sync with the real one.

Still not particularly impressed? Fair enough. Here is an experiment that shows just how strange things can get when we lose our usual sense of time. In 1962, French explorer Michel Siffre decided to find out the hard way. He spent around two months alone in a cave, with no sunlight, no clocks and absolutely no idea whether it was day or night. When he finally emerged on 14 September, he was convinced it was still August 20. He had effectively lost track of 25 days.

In everyday life, daylight resets the clock each morning. Without sufficiently strong light signals, though, our rhythm can start to wander. You might still wake at 7 am, go to work and follow your usual routine, while your body quietly insists it is living in a different time zone.

Of course, this doesn’t mean everyone who spends their days indoors is destined to drift. Artificial lighting can help regulate our clocks too, provided it is bright enough and timed correctly. Which raises a rather interesting question: if the brain relies on light to tell the time, does that light really have to come from the sun?

The short answer? To some extent, yes. And we are already doing it.

But don't rush to put all your faith in your regular IKEA lamp. Our biological clocks don't actually care whether light comes from the sun or a bulb. What matters is how bright it is, its spectrum and, perhaps most importantly, when we are exposed to it. So, while ordinary indoor lighting can influence our internal clock, it isn't always enough to keep it properly synchronised.

NASA has been working with this idea for years. And for good reason: on the International Space Station, astronauts experience around 16 sunrises and sunsets every 24 hours — hardly ideal for maintaining a regular sleep schedule. To help, the station uses specially designed LED lighting that can change in intensity and colour, providing brighter, cooler light during working hours and warmer, dimmer light before bedtime.

But you don't have to be an astronaut to benefit from carefully designed lighting. There are lamps made specifically for us ordinary Earth-dwellers. Known as light therapy lamps, they can deliver 10,000 lux of light at a specified distance — considerably brighter than ordinary indoor lighting — and are commonly used in the morning to help treat seasonal depression and adjust the body's internal clock.

But if you aren't getting much daylight and don't fancy adding yet another purchase to your list, there is some good news: you can start simply by changing how you use the lights you already have.

According to researchers, the trick is to give your brain a clear distinction between day and night. During the day, keep your space brightly lit, ideally using several light sources rather than a single desk lamp. Around three hours before bedtime, switch off the bright overhead lights and stick to one or two dim, warm lamps. And when it is finally time to sleep? The darker the room, the better.

There is even some science behind these simple changes: researchers recommend at least 250 melanopic lux at eye level during the day, dropping below 10 in the three hours before bed. As a rough guide, try several bright LED bulbs during the day, switching to dim, warm lighting (2200–2700 K) in the evening.

Still, there is one important distinction: replacing daylight's effect on the brain isn't the same as replacing everything sunlight does for the body. You might think I am contradicting what I said at the beginning, but here is the catch: ordinary indoor lighting doesn't provide the UVB radiation our skin needs to produce vitamin D. Without sufficient vitamin D from food or supplements, long-term deficiency can affect bone health.

For most of us, a few days without daylight is a minor inconvenience and a perfectly good excuse to grumble about the weather. But for researchers spending the winter in Antarctica, it is simply part of the job.

For several months of the Antarctic winter, the sun simply doesn't rise. Not once. The researchers get on with their work, but their bodies still have to figure out when to sleep, when to wake and how to stay alert without daylight’s usual help. Studies of Antarctic winter crews have been keeping a close eye on how that goes, and the results aren’t exactly reassuring. Disrupted sleep, constant fatigue, trouble concentrating and, in some cases, symptoms of depression: months without sunlight can take quite a toll. Then again, some people adapt surprisingly well.

The good news is that preventing at least some of the damage isn't all that complicated. And yes, we are coming back to much the same advice — but this time, there is some science to back it up. In one Antarctic study, just an hour of bright artificial light each morning helped researchers improve their cognitive performance and adjust their internal clocks. Add a consistent sleep schedule, regular exercise and a bit of socialising, and you have a decent starter kit for keeping body and mind on track, even when the sun is nowhere to be seen.

Most of us, of course, don't need to start living like polar explorers. But if your days are spent shuttling between underground car parks, offices and the sofa, you may recognise a few of the same ingredients. So, if your daily routine has become a journey from one artificially lit room to another, perhaps it is time to reconsider your relationship with daylight.