The body does not work to an exact 24 hour rhythm though. People have found that left to our own devices, we tend to work around a slightly different cycle. When we have access to the outside world though, we are reset daily by the sun. This prevents us from doing things at inappropriate times of day - or night!

How do biological rhythms work?

Since it is the sun which resets our rhythms each day, the assumption is that our eyes are the main organ through which this happens. Our eyes are stuffed with light sensing organs called rods and cones and also contain a light sensitive pigment called rhodopsin, so it seems reasonable to assume that these are the main light sensing organs.

However, other animals have light sensing organs elsewhere in their bodies. For example horseshoe crabs have sensors in their tails and eyeless fruit flies can still reset their clocks. Amphibians, birds and reptiles have light sensors deep in their brains and sunlight appears to be able to penetrate the skull.

But what of humans? Do we have another light sensing organ? Strangely enough it appears that we do. In a series of controversial experiments using bright lights shone onto the backs of volunteers knees, it appeared that our biological clocks can be reset this way. Do we have "eyes" in our skin? In laboratories all over the world, people are conducting a series of experiments showing that we seem to have light sensation in all sorts of body parts where they weren't supposed to exist. It also appears that we have clock genes which make us tick!

To make all this rather more plausible, as long ago as 1991 it was found that it is not the light sensing part of our eyes which resets our rhythms. Experiments with mice which have no rods or cones in their eyes showed that they were still able to reset their clocks, probably using ancient pigments called opsins. Along with these opsins, another set of pigments called cryptochromes seem to be involved. So the whole situation is a lot more complex than people first thought, and no-one is yet sure whether the opsins or the cryptochromes are the key to setting biological rhythms.

However, the light on the knees experiment is one of the most interesting, because it suggests that humans are totally different from other mammals. If true, it offers a way for people who need to reset their clocks to do so easily.

Why are biological rhythms important?

As a species we are what is called diurnal, that is, we sleep when it is dark, and wake when it is light. But in the modern world this is no longer possible for everyone. People working in factories and hospitals, airline pilots and train drivers, all need to be able to work through the night at some time. It generally takes time to reset your clock, particularly if you change your shift from day to night and back again over a relatively short period of time.

Imagine if all you had to do was have bright lights shone on the backs of your knees to reset your clock so that going to work at 7 pm did not feel all wrong?

At the moment we know that we can reset our biological clock by shining bright lights at peoples eyes, but that is not much use if you are trying to do this while in your normal diurnal state. If this could be done by shining bright lights against the backs of your knees while asleep, then this would be really useful.

Needless to say several commercial companies have already started developing lights for the back of the knees. However, as yet, the findings have not been repeated in another laboratory. So we are still some way off knowing if flying across the Atlantic with light-emitting jelly taped to the back of your knees really will make you able to cope with that morning meeting in New York.