The circadian rhythm taking hold is responsible for the feeling of fatigue. Our body naturally starts to shut down, around the time that the sun starts to take its dip into the horizon. The primary chemical for this happening is melatonin, so it would seem natural to wonder if melatonin has some effect on learning and memory formation as well. Every animal that sleeps also exhibits some bodily concentration of melatonin. When studying the effects of sleepiness on fish, it was found that any significant amount of melatonin causes a "dramatic decrease" in learning and memory formation.