Why is our circadian rhythm not 24 hours?

Can exposure to light change our body clock?
08 August 2017



Why is our circadian rhythm not 24 hours?


Chris asks Sarah Harrison from the University of Cambridge...

Sarah - I think you can define a circadian rhythm as any physiological process which occurs in cells on a 24 hour cycle or thereabouts. Each cell in the body has a set of genes which act as an internal clock making sure that every cell can keep to this 24 hour rhythm. In fact, the word circadian is Latin and roughly translated it means about a day. But, the question is absolutely right, the average human circadian rhythm is a little bit longer than that - its. 24.15 hours or thereabouts. Actually this is genetic variation so, individual by individual, every person has a slightly different length to their circadian rhythm. Some people people will have circadian rhythms a little bit shorter than 24 hours and some people have circadian rhythms that are a little bit longer. In fact, this genetic circadian rhythm can actually impact on your day to day behaviour and maybe Olivia will know a little bit more about this. If you’re a morning lark sort of person, so you’re good in the mornings, you can get yourself going - so certainly not me - then you tend to have a shorter circadian rhythm. If you’re a bit more of a night owl then then you tend to have a longer circadian rhythm.

Chris - So, Simon nighttime or owl I see you nodding there.

Simon - I used to be very much a morning person, and then I went to University and that slipped.

Chris - Yeah. I’m definitely an owl. I don’t function so well in the morning. Peter, are you a lark or an owl?

Peter - An owl.

Chris - Olivia an owl or off to bed early?

Oliva - Not an owl. Off to bed very early.

Chris - Isn’t that interesting. So why does that happen?

Sarah - Even though each cell has it’s own individual clock, all of these clocks need to be ticking with the same frequency if you like, otherwise, that would be no good to anyone. So the brain has a master clock in an area called the suprachiasmatic nucleus, and this master clock can be entrained by environmental light, essentially. It’s connected to the retina so it can sense light coming in from the environment and it’s also connected to a hormonal system so it can send a long range signal to all the cells in the body telling them essentially what time it is. As the questioner said, our circadian rhythm can be entrained to the 24 hour day and that’s why evolution has tolerated a little bit of variation either side of that because every day that you’re exposed to this environment light, that means that your circadian rhythm will be thereabouts 24 hours. If you take away these intrinsic cues, you essentially go and live in a house with no windows for a little bit, your rhythm will drift back to what’s been genetically coded, so it will slightly vary from this 24 hour cycle, and this is what’s called a free running circadian rhythm.

Chris - I think some blind people who lack the connection between the retina and the brain that you mentioned, they do have that don’t they, so their clock actually wanders with time, it slowly drifts off and they’re in a state of almost perpetual jet lag but then they do have an effect of trying to maintain a regular cycle and getting up at a certain time by setting an alarm clock, and that can help to keep them on track.

Sarah - It really can. I think there have been lots of people who want a better cure for jet lag. There was a study in the late 90s I think which showed, or seemed to show that if you shine a light on the back of your knees you could cure jet lag. As bizarre as this it is later turned out to be not the case. I think the only way to cure jet lag is to expose yourself to that early morning light and get yourself back in the rhythm.


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