June 2, 2007 Our body's biological clock influences a wide range of factors, including hormone levels, cognitive performance and sleep structure. Left to itself, this internal clock has an average periodicity of a little over 24 hours. For this reason, it has to be calibrated by external factors known as 'zeitgebers', the most important of which is the light/dark cycle. When the biological clock is disrupted, declines in cognitive performance and difficulties sleeping are often the result. New research, funded by the EU's Sixth Framework Programme (FP6) through the EUCLOCK project has found that exposure to brief periods of extremely bright light a few hours before sleeping is enough to synchronise the body's internal clock to the required day length.