Final answer:
The primary reason we sleep is not definitively known, but evidence suggests it is critical for cognitive function and memory formation, rather than merely clearing excess neurotransmitters or conserving energy.
Step-by-step explanation:
The question asks for an explanation of why we sleep. Among the proposed hypotheses, clearing the brain of excess neurotransmitters is one concept that has been considered, but it is not the sole reason for sleep. Other perspectives include sleep as an adaptive function to conserve energy or to provide safety from nocturnal predators, neither of which has been fully substantiated by research. However, the evidence that points towards sleep playing a crucial role in cognitive function and memory formation is noteworthy. Sleep deprivation has shown to impair decision-making, attention, and the recall of long-term memories, suggesting that sleep is vital for maintaining cognitive processes. The restoration theory advocates that sleep serves to repair and rejuvenate the brain and body, leading to the conclusion that sleep's primary purpose may cover a multitude of physiological and psychological processes.