Some of us are up like the larks and others are night owls who prefer a long sleep in the morning. Now scientists think they understand why.
"It is a remarkable fact that all life on Earth has some kind of biological clock," said Prof Russell Foster, of Imperial College London. These clocks allow us to "anticipate events" he said, but also to time activity inside individual cells so we are ready for action once awake.
Prof Foster and other researchers described the latest work on the "rhythm of life" and how our biological clocks regulate our lives during last week's British Association annual meeting in London. It provided a fascinating insight into the regulatory systems that tell us when to wake and when to sleep.
All life from insects to humans has an internal clock system because we evolved on a planet with alternating night and day. The system can easily be disturbed, however, as seen when we travel long distances and suffer jet lag.
Our bodies acclimatise to a given local time, Prof Foster said, and lock onto it. This connection is broken, however, when we move across time zones. "After a while our bioclocks lock onto the local time and exposure to local light is critical for this."
This adjustment to local time works for sighted as well as unsighted people he said. He described research linked to mutated mice which lacked the cone and rod cell structures in the retina that provide normal sight. These mice still adjust to local time, however. "If you don't have those rods and cones you can still regulate your clock," he said.
New work suggested that there was a wholly separate visual recording system not related to sight, he said. Researchers were beginning to examine cells imbedded inside the retina that seem to connect to the "clock centre" inside the brain which is associated with the hypothalamus. "It is clear the eye has a completely unexplored visual system," he said.
He cited the blind mole-rat, a mammal that lives in the deserts of Israel which in effect had no eyes and which spent its life underground. Despite this, the mole-rat was able to regulate its body clock, keeping it in tune with local time.
Dr Robert Lucas, also of Imperial College, described the molecular systems inside each cell that "make us tick". All life has this intercellular regulation system, and research had accelerated on this subject over the past two to three years, he said.
Scientists were tracking down the "clock genes" responsible for this process and there seemed to be nine in mammals, he said. The genes express proteins and these proteins in turn interact with other clock genes, stimulating some and inhibiting others. He said such a process could either lead to equilibrium or to oscillation - and in the case of the clock genes, it was oscillation.
The genes worked together, to regulate themselves, he said, but also to initiate actions within individual cells. The cells therefore "know" when the person is likely to wake. The clock genes begin switching on energy-production systems inside the cell, hormone signalling returns to waking levels and glucose levels rise so fuel is ready for use. All of this happens as we snooze comfortably.
The low point for most people comes at about 4 a.m., Prof Foster said, when body temperature and activity reaches its nadir. The clock genes pick up the pace some time after this in anticipation of waking.
Research suggested this low point is reached somewhat earlier amongst the human larks, meaning the wake-up call would come earlier. However, the owls among us don't reach their low point until somewhat later, making it harder for them to rise with the sun.
It could be difficult to convince the body to break out of its normal biorhythms, he added. Shift workers faced particular difficulties. Studies suggested that they are less alert than daytime workers, with the reduction comparable to having consumed two and a half pints of beer before going to work.