A gene could hold the key to sleep.  International researchers say a gene variant explains why some people get by on a few hours of shut-eye while others need a long time in bed to feel rested. Sleepers with one variant of the gene snoozed for longer periods than those with a different variant of the gene.  Scientists say the gene is one of several factors that influence sleep.  They cite the season, the age of the sleeper and the gender of the person who is sleeping as other factors in sleep length.