If a human or a prey animal is out and about at night, they are more at risk of being eaten. if the prey are in a safe place sleeping and conserving energy, they are more likely to remain unharmed. this reasoning is consistent with the of sleeping.

Some of the organisms have evolved in such a way that they remain inactive when the predator populations are active.  

Adaptive theory of sleep suggests that the prey population prefer to sleep when (in this case, at night) the predators are most active. The prey population use this time in taking rest and restoring energy for other works. As the prey population is inactive and remained protected in safe places, they are less likely to be predated.  

Hence, the correct answer is adaptive theory of sleeping.  

when atoms of different types of elements join together, they make molecules called compounds. water consists of compound molecules made up of 2 hydrogen atoms and 1 oxygen atom. this is why it's called h2o. water will always have 2 times the number of hydrogen atoms as oxygen atoms.