VIAGRA, as many men can attest, keeps the muscle that lines blood vessels in a relaxed state for longer than would otherwise be the case. The result is that the vessels remain open, blood-flow stays high and tissue remains taut. This happens because the drug inhibits an enzyme called phosphodiesterase that would otherwise mop up a chemical called cyclic GMP. Cyclic GMP's job is to tell muscle cells to chill out.

That is all well and indeed good. However, cyclic GMP is an all-purpose biochemical tool. It is also employed, for instance, in cells of the suprachiasmatic nuclei. These brain cells act as the body's internal clock. Some maintain a natural rhythm of roughly 24.5 hours (thus a human kept in the dark will gradually fall out of synchronisation with the day). Others entrain that rhythm to the light and dark cycles detected by the eye.

Diego Golombek and his colleagues at the National University of Quilmes, in Buenos Aires, realised the biochemical connection between Viagra and the circadian rhythm. They had previously shown that completely blocking cyclic GMP stops rodents from adjusting their internal clocks to time-zone changes. At least it does in one direction. Flip the daylight regime forward, as though the rodent is flying east, and cyclic GMP is necessary for the internal clock to adapt. Flip the regime backwards, as though flying west, and the chemical has nothing to do with making the adjustment.

With this insight, Dr Golombek and his colleagues decided to give their hamsters Viagra. Given that cyclic GMP helps shift the body's circadian clock, it seemed reasonable to Dr Golombek to ask whether manipulating the level of that chemical might change the rate at which the shift can happen. As they report in the Proceedings of the National Academy of Sciences, it does.

Hamsters are particular about when they climb into a wheel and begin running. Every day they start within five minutes of darkness. A hamster flown from Chicago to London would adjust gradually (as do humans). It would move its exercise schedule a little each day until it began running within five minutes of London darkness. Then it would stick with the new schedule.

Dr Golombek and his colleagues shifted their hamsters' daylight regimes by exactly the Chicago-London time difference—six hours—and injected Viagra on the same day. One group of hamsters received enough of the drug to develop erections. These took about six days to adjust their body clocks. Another group that was given a smaller dose took eight days to adapt. Hamsters that did not receive Viagra took 12 days.

This suggests—assuming humans react like hamsters—that half a little blue pill could help executives travelling from North America to Europe, or Europe to Asia, without the men among them developing certain side-effects. Women, perhaps, could hazard a higher dose.