we sleep well when we are in a familiar ,safe place,but we sleep lightly,if at all,when fear that bears will nose into the tent.
The evolutionary theory explains the difference in sleep among creatures.Why do cats,for instance,sleep so much,
while horses sleep so little?Surely cats do not need five times as much repair and restoration as horses do.
But cats can afford to have long periods of inactivity because they spend little time eating
and are unlikelyy to be attacked while they sleep.Horses must spend almost all their waking hours eating,
because what they eat is very low in energy value.Moreover,they cannot afford to sleep too long or too deeply,
because their survival depends on their ability to run away from attackers.