Are you feeling sleepy? Here's why... They are more likely to have hormonal imbalances, mood swings and trouble concentrating at work. Oh, and they are more likely to get fat, too. "If you are already a little chubby, and not very thin," Roenneberg tells me, "it is very likely that living against the body clock makes you obese." And how many of us are living these mistimed lives? "Two thirds of our society are woken up in the middle of our biological night," says Roenneberg. The way we live today, he says, is turning more and more of us into late sleepers, which makes the problem worse; increasingly, our body clocks want us to be owls (late to bed, late to rise), while our social clocks want us to be larks (up and ready at 6am). I'll come to that. But first, let's consider jet lag. Roenneberg is telling us that, these days, many of us have jet lag, even when we stay in the same time zone. We have jet lag when we wake up, we have jet lag when we potter about in our houses - and, importantly, we have jet lag at work, which has serious economic consequences. I know about jet lag. For decades, I've regularly travelled through time zones, finding myself in the middle of somebody else's day or night. I remember the first time it hit me really badly. Over the space of five days, I became increasingly unlike myself, sometimes inappropriately sleepy, sometimes full of manic energy. I'd flown for eight hours in a south-westerly direction, crossing several time zones. When I arrived, it should have been the middle of the evening, but it wasn't - it was early afternoon. So far, I thought, so good. By the evening I was sleepy. At dinner, I should have been in bed, entering my second sleep cycle. I fought this fatigue with cocktails. Finally, I went to bed and crashed out at midnight. But my body thought it was five o'clock in the morning. Two hours later, I woke up, wracked with tiredness, but unable to sleep. So I sat up in bed, reading and drinking coffee, for five hours. I got through a whole thriller. Full of caffeine, eye-poppingly awake but bone-tired, edgy and sludgy, I went to breakfast. I felt the ghost of last night's hunger, but didn't want to eat. "Coffee?" asked the waitress. I didn't really want it. But I drank it anyway. And towards the end of this time, something very embarrassing happened. In Internal Time, a brilliant book full of hard science, Roenneberg describes the many things that happen when you get jet lag. Of course, you get tired. "We are expected to be active when our body clock is on its way to bed, and we have to try and catch up on sleep when our internal alarm clock "announces" that it is time to get up." Also, we suffer "reduced alertness, poor motor co-ordination, and reduced cognitive skills." Our digestive hormones are released at the wrong time. When food is put in front of us, we're not ready for it. Our organs no longer work in synchronicity. The system misfires. You are out of whack - or, more literally, zoned out. During the day, mania tries to get the upper hand over torpor; at night, it's the other way around. In the end, you can't tell the difference. Starting with bacteria, says Roenneberg, every living organism evolved to have an internal clock, in order to find a workable niche. When cold-blooded reptiles roamed the Earth, they fed during the day, and huddled up at night, to get away from the cold. Mammals evolved in the opposite way; their niche was the night, although, as warm-blooded creatures, they were more adaptable. When humans evolved, fire was a big factor; we could sleep at night, protected by campfires, and forage during the day. It was important for some of us to be owls, and others larks, so that somebody was always alert enough to keep watch while other members of the tribe slept. It was an advantage for young adults to be late sleepers, so they, the best athletes, could hunt at night. In fact, young adults still tend to be late sleepers. But these days, society often thinks of them as lazy teenagers and students. They sit up late, hunting - not for animals, but for alcohol. Their lessons and lectures start early, when their cognitive function tends to be at its worst - which, says Roenneberg, is one of our society's big mistakes. He has argued in favour of later start times for German schools, even if this means rearranging the schedules of school buses. "Education is a nation's investment in its future," he writes. If there is a chance to increase the quality of such an important investment, then logistical problems, such as busing, have to be solved on the long view - even if this is difficult. Roenneberg points us towards experiments conducted by Mary Carskadon of the Brown University School of Medicine. She has taken students into a sleep laboratory in the morning, when they are supposed to be at school, and found that many of them "show the signs of a major sleep disorder - narcolepsy." In the lab they fall asleep, and enter REM, a sleep cycle "more typical of the end of sleep than its onset, showing that these students are physiologically still asleep, despite having gotten up in the morning." In the light of this information, a Danish school has recently cut out timetables; students were allowed to dictate their own start time. Roenneberg says he's keen to see the results of this project. Meanwhile, he is organising a huge survey "that aims to generate a world map for sleep-wake behaviour." Of course, nobody had heard of jet lag until the arrival of jets - if you sail across the Atlantic, your body clock has time to adjust. Still, nobody worried about the concept of time zones until the invention of trains. Before that, everybody set their clocks by the sun. But when you travelled by train, you had to keep resetting your watch. So in 1884, everybody agreed to divide the world into 24 time zones. Since then, our internal clocks have been at the mercy of the onward march of our external clocks; our relationship with the sun is getting weaker all the time. But it still persists; in a survey conducted by Roenneberg himself, Germans were studied according to "chronotype" - whether they were early or late sleepers. Every morning, it takes the sun 36 minutes to pass over Germany - four minutes for every line of latitude. It turned out that the further west a German lives, the more likely he or she is to be a late sleeper - even though Germany is contained within a single time zone. Which tells us an interesting thing: our bodies want to live according to the sun, rather than the alarm clock. What Roenneberg is really saying is that we should think much more about our body clocks, because it's not just what we do that's important, but when we do it. Timing our sleep is key. When I track Roenneberg down, he's in Pensacola, Florida, at a body clock conference. "Modern society regards sleep as superfluous - something we should get rid of," he says, in his calm, precise German voice. As if we need a cure for sleep. And the thing is, sleep is not one state - it's many different states. Sleep is as heterogeneous in its functions as the wake state, but we are not aware of it, he says. If you get the right amount of sleep, he says - in other words, if you go to sleep and wake up according to your own body clock - "which hardly anybody can, you would realise that the quality of the other 16 hours is increased enormously." Roenneberg himself likes a good eight hours - although, at 58, he says he sometimes wakes up and reads. I ask him if he sleeps alone. "I don't think that's any of your business," he says. Then he adds: "That was cheeky; it wasn't meant sternly." He doesn't want to say whether he's a lark or an owl, so his readers don't think he's biased towards one type or the other. Short-sleeping larks, he says, have an advantage when it comes to making money. The commercial world wants you to wake up early, and it's an advantage not to need too much sleep. But the problem is that more and more of us are becoming owls. The world we live in is making us want to sleep later and later - hence the prevalence of alarm clocks, and the increase in social jet lag. But why is this? It is, he says, because the modern world - the world of alarm clocks and offices - puts us out of touch with the Earth's rotation. We spend our days indoors, under weak lights, and at least part of our nights in similar conditions. Relatively speaking, the modern world is a world of dim days and bright nights. This is nothing like the natural world. "The differences are huge," says Roenneberg. In the fake, modern world we don't get enough light to tell us that the day has truly happened, and the nights aren't dark enough to send us to sleep. "Our body clocks are not getting a clear signal any more," he says. For all the functions within the body, the clock on the wall is absolutely meaningless. And that's why we get jet lag. That's why the embarrassing thing happened to me on the fifth day of my trip. I was having lunch on a terrace outside a restaurant. I nodded off. When I woke, I didn't know how long I'd been asleep. But then I had an overwhelming urge to fall asleep again. I made a hasty plan. I would escape. My cognitive processes were awry. I walked away from the table, hoping I could get out of sight before I collapsed. I only managed to get a few yards. I lay down under a tree, and blacked out. When I was woken up, two minutes later, I had no idea where I was. In the battle between the clock on the wall and the one in my head, my internal clock won hands down.