A body-clock "atlas" mapping the 24-hour activity patterns of thousands of genes has been created by scientists.
The five-year-project is expected to provide important clues about the way drug effects may be influenced by timing - for instance, taking medicine in the morning or at night.
The research showed that most best-selling medicinal drugs target proteins made by genes whose activity changes daily.
Timing is an important but often ignored factor in drug effectiveness, the scientists point out.
Dr Michael Hughes, a member of the team from the University of Missouri-St Louis in the US, said: "The genome is under much more clock control than we once thought.