Everyone on Earth marches to the same beat: we have an internal body clock that keeps us on a 24-hour cycle. It’s fundamentally important for our sleep cycle, but it’s also crucial for our general health and well-being, and is linked to everything from our hunger and metabolism to our heart function, mental health and immune system.


Studies have linked a disrupted body clock to a greater risk of diabetes, heart disease and cancer. With this in mind, we caught up with Ella Al-Shamahi, an evolutionary biologist and presenter of the Horizon episode Body Clock: What Makes Us Tick?, to find out how we can hack our sense of time and why she locked former Commando Aldo Kane in a nuclear bunker for 10 days…

Why did you lock someone in a nuclear bunker?

Because we live in this modern, technological world, we don’t really realise how powerful our body clock is, nor the factors that affect it. So the idea was: put a Commando [Aldo Kane, a former Royal Marines Commando] in an underground nuclear bunker, with no access to sunlight and no way of telling the time, and control his access to artificial light… and see what that tells us about our body clocks.

What exactly is the body clock?

It’s our internal clock that keeps all our body functions in sync. It’s regulated by a tiny region in the brain located in the hypothalamus, and it takes its cues from the day-night cycle of sunlight. The brain uses nerves and hormones to transmit this 24-hour rhythm to our internal organs, which helps to tell our body when it needs to eat, sleep, wake and work.

As an evolutionary biologist, the really interesting thing for me is that the body clock is a highly ‘conserved’ mechanism, which means that it’s stuck around for a long time in evolutionary terms – millions and millions of years. If something is highly conserved, that usually means it’s pretty useful. Even fruit flies have a 24-hour body clock.

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What did the bunker experiment involve?

There were three phases. In phase one, for the first few days, we didn’t do anything other than just put Aldo down there and monitor him while he went about a normal daily routine – eating, sleeping, exercising, reading. We didn’t give him any indication of what time it was. We were doing shifts monitoring him above ground, so we communicated with him via a tapping mechanism so that he wouldn’t be able to work out what time it was by who answered the phone.

Former Commando Aldo Kane volunteered to be shut in a nuclear bunker without access to natural light for Horizon’s body clock experiment © Windfall Films/Robert Hollingworth
Former Commando Aldo Kane volunteered to be shut in a nuclear bunker without access to natural light for Horizon’s body clock experiment © Windfall Films/Robert Hollingworth

He wasn’t getting any natural sunlight, of course, but he could control switching the lights on and off within the bunker, so when he woke up he’d put all the lights on, and then when he went to sleep, he’d switch them off again.

The second phase of the experiment was what we called ‘going dim’. We turned off all the lights, and we left only a dim lamp, so he was pretty much in darkness for a few days. We wanted to find out how the body clock coped when there was no light at all.

And then in the third phase we sent Aldo into jet lag mode. Usually, when you go into jet lag, perhaps when you’re flying from New York to London, you have one disrupted sleep, but then you make up sleep over the next few days. But we woke Aldo up in the middle of his sleep at the same time for several days in a row, keeping him in constant jet lag. We weren’t really giving him a break.

How did he cope with all this?

Not well! He was losing track of time – he thought it was a completely different time to what it actually was. And in the third stage, we were forcing him to wake up, so that kind of messed with him. He’s someone who’s tough, which is why we chose him for the experiment, but he was clearly losing it. It was partly the lack of contact with other people. But it was also clearly the fact that his body clock was out of sync. He was really miserable by the end of it.

What did you find out?

In the first phase, even though Aldo had no way of knowing the time, his body clock still broadly kept to a 24-hour cycle. He wasn’t suddenly shifting to 36 hours, or 12 hours for that matter. So it’s not your watch, or your phone, or the outside environment that’s controlling that – your internal clock keeps its own time. But we could see that his sleep was shifting later and later.

When we turned off the lights in phase two, we saw that shift in Aldo’s sleep pattern even more. Without any light, his body clock really struggled to keep time – it entered a stage called ‘free running’, which I think is a really great term. Essentially it’s where the body clock runs away with itself. We aren’t sure why, but most people’s clocks run slightly longer than 24 hours, and this is what we saw here. We need light to ‘reset’ and recalibrate our body clock.

Just as our bodies have an overall ‘master clock’, different organs also have their own clocks. So your ability to do certain things is governed by the time of day.

I asked Aldo before he went in what time he normally woke up, and he said 6 o’clock every morning, with or without an alarm. I thought that was really interesting because here was somebody whose body wakes him up at 6 o’clock every day, but suddenly, when we took away his access to sunlight, he was shifting later and later. By the end of the experiment he was over three hours out of sync with the outside world. It’s like those old winding clocks. You need to recalibrate them every day just to make sure that they’re telling the right time.

During the film, I also meet a man called Mark Threadgold who lost his sight while serving in the British Army. Most blind people have some kind of light perception, but Mark’s optic nerve was severed, so he doesn’t see any light at all. He is constantly in that free running phase. Every day, he loses about an hour of sleep, so in the space of a month he does a full circle. He described how lethargic he was, and how it was really bad for his mood, so it really brought home the kind of impact this can have.

How can we apply these findings to our own lives?

What we put Aldo through was an extreme situation, but modern living is also quite extreme from an evolutionary perspective, in that a lot of us spend our days without much natural sunlight, which is something we’re not designed for. So some of the advice is really practical. Maybe you can cycle to work instead of getting the train, or take a little walk outside during your lunch break.

Another interesting thing I found from talking to the researchers in the programme is that, just as our bodies have an overall ‘master’ clock, different organs also have their own clocks. So your ability to do certain things is governed by the time of day. We saw this with Aldo in his tests in the bunker.

I had always thought that you’re supposed to work out first thing in the morning but, the truth is, I’ve never really been much good at doing that. It turns out that’s not just me. In the morning, our bodies are still waking up, so it’s best to wait until later in the day to work out. The morning is a good time to eat a big meal, though, because our metabolisms are more efficient then. It’s better not to eat a big meal at the end of the day. So our digestive systems have a body clock, too.

Is there any way to completely reset your body clock?

In the Horizon programme, I meet a couple who are struggling to synchronise their body clocks. Naomi is an early bird, while Greg is a night owl. They go to sleep together, but then Greg just faffs around in bed for hours until he finally gets to sleep. They really want to address that difference, because it disrupts Naomi’s routine too, and they are getting married.

In a way, Greg is completely normal. Around 25 per cent of the population are night owls, while 25 per cent are early birds, and the rest are somewhere in between. We can’t radically change where on the early bird/night owl spectrum we fall, but it is possible to shift our body clocks in a particular direction. Greg wanted to shift his body clock a bit earlier to match his partner’s.

So the sleep scientist in the programme gave him goggles that cut out blue light in the evenings [these fool the body clock into thinking it’s darker than it is]. In the old days, if you wanted to work into the night, you had to light a candle. So you were already preparing yourself to go to sleep, whereas now you can have bright lights on up until the very second that you fall asleep.

Greg also started going outside more during the day to get that sunlight. If you’re a night owl, you want to be getting outside in the morning sunlight, whereas early birds can shift their sleep a bit later by getting more sunlight in the afternoon. With Greg, the changes really had a positive impact.

What's the best way to reset your body clock? © Getty Images

What other body clock tips did you pick up?

To help your body clock keep a regular rhythm, aim to go to bed and wake up at around the same time every day. Try not to use your phone at night, but if you have to, use a night mode or blue light filter.

I meet another couple in the programme who both work night shifts – one of them really struggled to go to sleep, and the other one would sometimes sleep for 24 hours in one go. We sat them down with a researcher, who explained that they really need to be sticking to the same sleeping pattern every day, even on their days off. Keeping to the same timetable is much better for the body clock.

Something else that came up while making this film is the fact that most jobs are nine-to-five. But that ‘one-size-fits-all’ model is just not going to work for some people. If you’re a night owl, it’s no good expecting to be productive at 8 o’clock in the morning. It’d be great to move to a society where people can be flexible in their working times so that they can make their schedule work for them.

Have you changed any of your own habits?

My sleep is generally terrible and I don’t respect my body clock at all, so I became slightly mortified at the impact I was having on my own health. I’m not a shift worker, but my sleep resembles one – I work really strange hours. So I’m now trying to stick to the same sleep routine every day. I’m trying so much harder since I did the show.

  • This is an extract from issue 328 of BBC Focus magazine - subscribe and get the full article delivered to your door, or download the BBC Focus app to read it on your smartphone or tablet. Find out more


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James Lloyd
James LloydStaff writer, BBC Science Focus

James is staff writer at BBC Science Focus magazine. He especially enjoys writing about wellbeing and psychology.