If you want to keep a healthy weight, sleep seven to eight hours a day and no more or less, scientists have discovered.

A new study conducted by obesity researchers at Laval University in Quebec City suggests that seven to eight hours is just the right amount of sleep, with people who get too little and those who get too much actually putting on weight as a consequence.

The study showed that people who slept five to six hours a night and people who averaged nine to 10 hours a night put on more weight when followed over a six-year period as compared to people who got the recommended seven to eight hours of shut-eye.

The researchers followed 276 adults aged 21 to 64 years, over half of whom were drawn from families where at least one parent and one offspring were obese by BMI measures.

The result showed that short sleepers gained an average of 1.98kg more than average sleepers and long duration sleepers gained an average of 1.58 kg when compared to the optimal length sleepers.

As to why those who were getting too little or too much sleep were more likely to gain weight, the study cannot provide definitive answers. But senior author Angelo Tremblay of the study said the thinking is that sleep deprivation disrupts the production of hormones that regulate the body's appetite. Too little sleep appears to increase production of ghrelin, the hormone that tells us we are hungry, and decrease production of leptin, the hormone that tells us we are full.

As for long sleepers, Tremblay said some researchers believe they are actually bad sleepers, and that they stay in bed for longer because they have not had high quality sleep.

The study, published Tuesday in the journal "Sleep," fits with a number of recent scientific articles that suggest a U-shaped relationship may exist between sleep duration and body mass index, the height-to-weight measure used to differentiate between healthy and unhealthy weights, scientists have pointed out.

(Agencies via Xinhua April 2, 2008)