Since elephant seals are submerged 90% of the time they are at sea and they are at sea 8 to 10 months of the year, depending on sex, when do they sleep? They hold their breath while sleeping on land, as if they are diving. This suggests that they do not sleep at sea during the short periods at the surface. A good case can be made that they sleep during dives. This is reasonable because heart rate slows and metabolism decreases during diving just as it does during sleep. On land, elephants sleep whenever they can, day or night, that is, when the entire colony is quiet.