Are You a Lark or a Night Owl? What your Sleep Habits Reveal About your Health

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You’re up at the crack of dawn, raring to go, while your other half is dead to the world. Then, while you’re ready for lights out at 10pm, they’re happy to burn the midnight oil… and some. 

Sounds familiar? It’s the difference between a lark and a night owl. And it won’t just affect your social life, for researchers are discovering these characteristics have implications for health, too. 

This preference for morning or evening is known as your sleep chronotype, and it affects our waistline, fertility, pain levels and even cancer risk. It also affects personality — a study published last month found night owls are more likely to demonstrate dark personality traits including narcissism and deceitfulness.

Researchers from Sydney and Liverpool interviewed more than 200 people about their personalities and sleeping habits. They suggested the selfishness of night owls might be an evolutionary hangover, because such people are more likely to scheme and steal sexual partners from others, which is best done under cover of darkness.

Whether you have a morning or evening chronotype is dictated by your biological 24-hour clock, explains Dr Tim Quinnell, from the Sleep Laboratory at Papworth Hospital, Cambridge. This, in turn, is heavily influenced by genes.

‘Everything in the body — every reaction, hormone, gene switching on and off — is governed by the internal clock,’ he says. ‘And it’s this clock that makes early types wake when they do, and late types able to carry on into the night.’ 

Here, the experts reveal the latest research on owls or larks, and the effect on health.


Being a night owl or lark may be largely dictated by a gene known as Period-3. Scientists at the University of Surrey discovered there are two versions of this gene — a long version and a short version. Those with the long version are larks; the short version, owls.

The gene is thought to affect ‘sleep pressure’. As well as our biological clock controlling when we sleep and wake, we also have a system that builds up feelings of sleepiness throughout the day — the peak is when we are at our most tired and need to go to bed. The Period-3 gene causes sleep pressure to affect larks and owls differently, explains Dr Simon Archer, reader in chronobiology at the University of Surrey. 

‘The larks have a sleep pressure that builds up much more quickly. So as they go through a normal day, they get more tired more quickly.’

We each carry two versions of the Period-3 gene — one from each parent. If you get two versions of the long or short version, you will be an ‘extreme’ lark or owl. 

Many of us have one version of both, meaning we have tendencies for characteristics of both, says Dr Archer. ‘I tested myself and found that I have one short gene and one long gene. This makes sense, as I work best in the morning, but I have the physiology of an owl and so I can’t eat breakfast first thing.’ 


Larks always eat breakfast within half an hour of waking, says Professor Jim Horne from the Sleep Research Centre at Loughborough University.

‘We’ve found this is a very good indicator of whether a person is a morning or an evening type,’ he says.

This might be because our body clock influences metabolism.

But owls are more partial to a midnight feast. In a recent study on 119 obese volunteers, half who were morning types, the others evening types, the latter consumed twice as many calories after 8pm — on average 677 calories, compared with 299 for larks. 

Furthermore, the morning types had their breakfast around 7.17am — the evening types ate at 8.38am. 

The problem for owls is that evening meals may not be as filling as day-time meals — leading to over-eating and weight gain. 

This may be due to low levels of leptin, a hormone responsible for telling our brain when we are full.

According to sleep expert Professor Russell Foster from Oxford University, research has shown levels of this hormone can go out of kilter when we’re sleep deprived. 

Owls tend to be more sleep deprived than larks as they go to bed late yet have to wake early for work.

‘One study revealed that even short-term sleep deprivation — seven days of four hours’ sleep a night — resulted in carbohydrate consumption, particularly sugar, up by 35-40 per cent.

‘The ability to clear glucose from the blood was bordering on diabetic,  and levels of the hormone leptin were down by 17 per cent.’

Late-night snacking means owls tend to be larger than larks. A recent study published in the journal Chronobiology International found owls had greater weight gain than morning types.


All this night eating may affect the owl’s overall health. In a small study of 11 people, conducted by the University of Pennsylvania, evening types had lower levels of ‘good’ HDL cholesterol (the other types of cholesterol were not monitored).

Owl types were also more likely to snore and suffer from sleep apnoea, where breathing stops for periods of at least ten seconds at a time. 

The researchers said that the fact evening types were overweight may contribute to this — the condition is linked to fat around the neck. 

The study also showed that evening types had higher levels of stress hormones, which may exacerbate the condition.


Some studies suggest that larks are at greater risk of some cancers, particularly breast and colorectal cancer. This seems to be connected to the longer version of the Period-3 gene — and many larks carry two copies of these. 

Another theory is it’s linked to melatonin, the hormone crucial for sleep — high levels are released when we’re in the dark, and low levels in light. 

Dr Archer suggests morning types may have more exposure to more light, so may have less melatonin. 

Some studies have suggested that melatonin has antioxidant properties and may even protect against cancer. 

However, Dr Archer says staying up at night under artificial light can also stop the melatonin, so more work is needed to understand exactly what is going on.


Studies suggest that because owls tend to go to bed later, their sleep ends prematurely, says Dr Quinnell. 

Sleep has a number of distinct phases, including around four REM (or dreaming) phases. But because owls go to bed late and wake up early for work, they often don’t have their last phase of dreaming sleep, which may affect memory. 

‘This phase of sleep helps the brain lay down memories and runs through the experiences of the day,’ says Dr Quinnell. ‘It de-briefs the brain and helps us to learn from experiences. It’s more healthy for the body and the mind to have all the stages of sleep.’

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