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.
Larks feel more tired
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.'
Owls are hungrier - and fatter
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.
Owls are more likely to snore
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.
Larks may be more at risk of cancer
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.
Owls may have poorer memory
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.'
They may also suffer more pain
Their lack of deep sleep could leave owls in pain — a small U.S. study found people deprived of REM sleep are more sensitive to pain the following day.
Furthermore, while many over-40s wake up with joint pain and stiffness, being an owl could make this worse, suggests Dr Chris Edwards, a consultant rheumatologist at Southampton General Hospital.
At around 4am our body releases a burst of natural anti-inflammatory molecules, which can ease inflammation in the joints, reducing pain and stiffness. However, while larks may wake in time to feel the effects of this burst, it may have worn off by the time owls rise.
OWLS ARE FUN BUT RISK DEPRESSION...
On the whole, owls seem to have more fun. Research suggests evening types tend to be sensation-seeking, risk-taking and more outgoing.
One study of more than 800 people published in the journal Chronobiology International found larks were better at controlling impulse. Another study of more than 1,000 people found they were also more likely to be agreeable and conscientious.
They tend to be more analytical, while evening types think more laterally, says Professor Horne.
Being an owl may also raise the risk of depression, says sleep expert Dr Neil Stanley — possibly because of lack of sleep.
It could also make symptoms of depression worse — a recent study of 100 people with depression in the Journal of Affective Disorders found that evening types experienced more severe symptoms, including anxiety.
Dr Quinnell adds that long-term REM sleep deprivation can lead to hallucinations, as the brain starts to have 'dreaming episodes' when it is awake.
... YET THEY HAVE A LIVELIER LOVE LIFE
Several studies have shown night owls tend to have more partners. One Durham University study of 106 men aged 18-30 suggested that male morning types have an average of 3.6 partners, while evening types have an average 16.3 partners.
Aside from the obvious advantage of having a more active social life, biology may also be at play.
A study last year by German scientists of more than 100 men between the ages of 19 and 37 found that night-owl men tend to have high testosterone levels, possibly because levels of this hormone are linked to our sleep-wake cycle.
Other studies have shown that evening types tend to have more children. But when it comes to women, one Finnish study published last month of more than 2,000 females suggests that morning types could be the most fertile, which may be linked to the hormones that govern the menstrual cycle and ovulation.
WE ALL GROW MORE LARK-LIKE WITH AGE
Although our pattern is largely set by genes, it alters depending on age, says Professor Foster.
'After the age of ten, we go to bed later and get up later. This 'owlness' peaks at the ages of 20-21 for men, and around 19 for women.'
Teens are natural owls, which is why parents should let them sleep in at the weekends, says Till Roenneberg, professor of chronobiology at the University of Munich.
'Ideally 16 to 18-year-olds should not start school before 10am. But they do, and so are sleep deprived come Saturday.
'Let them lie in, but make sure there is some light in their bedroom in the morning as they sleep, otherwise their body clock could shift even later.'
After the teen years, men are generally more owl-like than women, going to bed and rising later. This difference is down to the sex hormones oestrogen and testosterone.
But as these hormones start to decline, we all become more like larks than owls, says Professor Foster. This change can be particularly acute in women around the time of the menopause with the dramatic drop in oestrogen.
IT'S HELL WAKING UP AT 4 AM
Unfortunately, this transformation to a lark can lead to some elderly people being wrongly diagnosed with depression, says Professor Horne.
'As people get older, they can find themselves waking earlier and earlier — especially if they were already larks. People can start to be wide awake as early as 4am, and they start to fret about this.
'They go to the doctor, and because early waking is also a sign of depression, they may be prescribed antidepressants.
'But these can make people tired in the day, and more awake at night, which just exacerbates the problem.'
To counteract early morning waking in old age, Professor Horne recommends having a cup of coffee at 9pm and then napping for 15 minutes before the effect of the caffeine kicks in.
You should then be able to stay awake until midnight. This will delay your sleep — after a week, you should be waking at 6am rather than 4am.
WHY OWLS SHOULD MARRY LARKS
'There is some evidence that the longest surviving marriages are between morning people and evening people,' says Professor Foster.
'One explanation is that if you can accommodate your partner's sleeping habits, it shows true give-and-take in a relationship.' He added: 'My wife is a morning person, but I am a definite owl — I go to bed with a torch to read. We worked that out early on and it's worked ever since.'
Professor Roenneberg adds that partners should be more tolerant of each other's sleep habits.
He says that trying to work against your chronotype — getting up early if you are an owl or staying up late if you are a lark — can lead to 'social jet lag', where you are constantly fighting against your natural body clock.
This can increase inflammation in the body and heighten risk of a range of illnesses, including diabetes, obesity and depression.
He adds: 'Never criticise your partner's sleep pattern.
'Let them lie in or go to bed early — they are not being lazy or boring.'
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