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.  

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genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).