Are you a lark or an owl?
Whether you prefer being up at dawn or burning the midnight oil depends on your genes, experts have found.
Some of us leap out of bed each morning, raring to start the day. Others need at least one alarm clock - preferably one with a snooze button - to ensure they get to work on time.
And some of us happily stay up chatting until the wee small hours, while others prefer to be tucked up listening to 'Book at Bedtime' with the lights turned out.
We really are divided into larks and owls. And this is set by our genes, says neurogeneticist Dr Louis Ptacek of University of California.
He says: "Whether we like it or not our parents are telling us when to go to bed - based on the genes that they gave us."
Scientists have come to realise the importance of understanding a person's chronotype, the time of the day when they function the best.
Knowing how much of a lark or an owl we are should help us live more healthily in the modern 24/7 world.
Rick Neubig, a professor of pharmacology in Michigan, is an extreme lark.
"People I communicated with in Europe will always notice that they get emails from me very early in the morning.
"The other thing I like a lot which fits in with the early mornings is that I'm a fairly serious bird watcher. It's much easier for me than other people to get up and see the birds at dawn."
And it runs in his family.
"My mother would always drag us out of bed at 4 in the morning to go on vacation, and my daughter works out early in the morning."
'Strong genetic trait'
Dr Louis Ptacek is studying the families of larks like Rick's that have Familial Advanced Sleep Phase syndrome. He got into this area of research when his colleague Dr Chris Jones met a 69 year old who was worried about waking up very early and whose concern had been ignored by other medics.
Drs Ptacek and Jones looked at her family.
"We recognised this was a strong genetic trait. We found the mutated gene resided near the end of chromosome 2", says Louis Ptacek.
They knew that if similar genes were mutated in fruit flies and mice the circadian clocks speed up. The mutated gene made a different protein that affects the rhythm of the clock.
They also study families of extreme owls, with Familial Delayed Sleep Phase syndrome. And they think this was due to a different mutation in the same genes.
Mutations in other genes have been found in other families with advanced or delayed sleep patterns.
We all have internal circadian clocks - the master clock is made up of thousands of nerve cells in the suprachiasmatic nucleus, a wing - shaped structure located in the hypothalamus, at the base of the brain.
The hypothalamus controls all kinds of bodily functions, from releasing hormones to regulating temperature and water intake.
This internal clock is reset every day by light. You might expect that since the earth's day lasts 24 hours, everyone's clocks would run to a similar schedule.
But they don't. That's why there are larks and owls.
"If you have a fast clock you like to do things early, and if you have a slow clock you like to do things late," says Prof Derk-Jan Dijk, Head of the University of Surrey's Sleep Research Centre.
'A sleep map of the world'
Our clocks are not fixed throughout life. Anyone who has small children will know they're prone to waking early, as do the elderly.
But whatever the speed of your clock we have to fit in with the way that society is set up with its 9-5 working times.
This can be particularly hard for teenagers, who generally find it hard to get up in the morning.
Prof Till Roenneberg of Ludwig-Maximilians University has looked at the sleeping patterns of this age group with the help of his Munich Chronotype Questionnaire.
"We can show that the famous lateness of teenagers is a real thing. They get later through childhood and puberty and reach a point of lateness at 19 and a half for women and 21 for men. It was so clear it was astonishing.
"Our database has over 200,000 participants. We are hoping for a sleep map of the world."
Mary Carskadon, a professor of psychiatry at Brown University in the US, is campaigning for schools to start later.
"School grades don't get always higher but for me one of the most important aspects of sleep loss is the issue of depression and sadness and lack of motivation of kids.
"The moods improve when schools start later."
But not many schools around the world have chosen a later start time.
After all, most people do fit in with the working day, although they may be suffering from exhaustion.
Social jet lag
Prof Roenneberg has a catchy way of describing and measuring the sleep deprivation many suffer during the working or studying week, when we rely on alarm clocks to get us out of bed.
He calls it social jet lag.
He finds that the middle of people's sleep on work days is usually earlier than that on free days. The difference is their social jet lag.
"On average people accumulate one to two hours of social jet lag, though some can get up to five hours, particularly in the young, who still have to get to work at the same time as older people," says Prof Roenneberg.
Having social jetlag is like flying from New York to London every weekend. And it's harder to get over social jet lag than time zone jet lag.
But Prof Roenneberg says there are things we can do to overcome social jetlag.
"We should be changing work times and making them more individual to fit in with our chronotypes. If that's not possible we should be more strategic about light exposure.
"You should try to go to work not in a covered vehicle but on a bike. The minute the sun sets we should use things that have no blue light, like computer screens and other electronic devices."