Should we sleep and wake early to boost our health?
When a recent study looked at health differences between late and early risers, it appeared to make grim reading for night owls.
Increased risk of early death, psychological disorders and respiratory illness were the stark findings from the paper, which backed up other research suggesting late-nighters are more likely to suffer ill health.
But is being a night owl really bad for you and does it mean some of us should ditch the late nights and lie-ins to become more like morning larks?
'Social jet lag'
It's a scenario familiar to many workers during the week.
After struggling to fall asleep, you're abruptly dragged out of your precious slumber by the jarring siren of the alarm clock.
By the weekend you're exhausted and sleep way past your Monday-Friday waking time to catch up on some precious sleep.
This may sound perfectly normal but it's a sign not only that you're not getting enough sleep but also that you have "social jet lag".
This is the term for the difference between when we sleep during the week compared with the weekend, when we're free to go to bed and get up at the times we like.
The bigger the social jet lag, the greater the health issues, such as increased risk of heart disease and other metabolic problems.
This is what is driving those studies that find night owls - particularly very late risers - are at increased risk of ill health compared with their morning-loving counterparts, according to Till Roenneberg, professor of chronobiology at the Institute of Medical Psychology at Ludwig-Maximilian University in Munich.
With many jobs and schools often having early start times, night owls are effectively having to operate in morning lark time.
If you forced early risers to have to work late into the night they'd also face health problems, says Russell Foster, head of the Nuffield Laboratory of Ophthalmology and the Sleep and Circadian Neuroscience Institute.
'It's human biology'
So how should night owls cope?
Should they sacrifice their lie-in on the weekend and set the alarm early to get back into sync with the time they have to operate in during the working week?
"It is the worst thing you can do," says Prof Roenneberg, who believes there is nothing inherently unhealthy about being a night owl.
"If you haven't slept enough for five days you'd better catch up on your sleep and you better try to catch up at your individual best time to do that, and that's later."
This is because when we want to sleep and wake is not just a habit, nor is it a sign of discipline.
Instead, it is influenced by our body clocks, about 50% of which is determined by our genes.
The rest is shaped by our environment and age, with 20 being around the peak age of lateness and our body clock getting progressively earlier as we get older.
"We have these ingrained attitudes that people who stay up late must be up to no good and people who sleep in late must be lazy, but really it is human biology," says Malcolm von Schantz, professor of chronobiology at the University of Surrey.
This results not just in larks and owls, but a spectrum of body clocks, ranging from very early to very late and others in between.
Simply getting up earlier is unlikely to override your genetic tendencies and instead will further deprive you of the sleep you're already not getting enough of in the week, experts say.
Instead, a better way for night owls to manipulate their body clocks to be more like larks is through changing habits around when they are exposed to light.
Our body clocks are influenced by the rise and fall of the sun, but many of us get little sunlight in the day and too much artificial light at night.
This delays when we feel sleepy - a particular problem for night owls whose biology already predisposes them to lateness.
By getting sunlight in the morning and reducing artificial light in the evening - particularly from things like phones and laptops which produce powerful blue light - we can train our body clocks to feel sleepy earlier.
But it's not an easy process for everyone, in part because many of us struggle to get enough sunlight in the day and find it hard to limit our exposure to artificial light in the evening.
'Task of society'
Instead, it is workplaces, schools and society more generally that need to do more to accommodate night owls, sleep experts say.
At its most basic level, more employers should allow evening types to start and finish work later.
Beyond this, Prof Foster says, it would make sense for some workplaces to let staff work hours that match their individual body clock.
This would boost employee performance and be a more effective way of staffing 24/7 businesses.
But Prof Roenneberg goes even further, arguing that society has a duty to fix the sleep-disrupting environment it has created.
"It is the task of society to take care of this, the task of society to make more light in buildings, and to give less blue light to give people the possibility to watch TV without setting their clocks."
