Although largely unrecognized by sleep scholars, sleeping is a pleasure. This report aims first, to fill the gap: sleep, like food, water and sex, is a primary reinforcer.

The levels of extracellular mesolimbic dopamine show circadian oscillations and mark the “wanting” for pro-homeostatic stimuli. Further, the dopamine levels decrease during waking and are replenished during sleep, in opposition to sleep propensity. The wanting of sleep, therefore, may explain the homeostatic and circadian regulation of sleep. Accordingly, sleep onset occurs when the displeasure of excessive waking is maximal, coinciding with the minimal levels of mesolimbic dopamine. Reciprocally, sleep ends after having replenished the limbic dopamine levels. Given the direct relation between waking and mesolimbic dopamine, sleep must serve primarily to gain an efficient waking.

Pleasant sleep (i.e. emotional sleep), can only exist in animals capable of feeling emotions. Therefore, although sleep-like states have been described in invertebrates and primitive vertebrates, the association sleep-pleasure clearly marks a difference between the sleep of homeothermic vertebrates and cool blooded animals.

尽管睡眠学者基本上没有意识到，但睡眠是一种享受。该报告首先旨在填补空白：睡眠，如食物，水和性生活，是主要的补充。

细胞外中溶边缘多巴胺的水平显示出昼夜节律性振荡，并标志着稳态的刺激“想要”。此外，与睡眠倾向相反，多巴胺水平在清醒过程中降低并且在睡眠过程中补充。因此，睡眠不足可以解释睡眠的体内平衡和昼夜节律调节。因此，当过度醒来的不适最大时发生睡眠发作，这与中脑边缘多巴胺的最小水平相吻合。相应地，补充边缘性多巴胺水平后，睡眠即告结束。考虑到清醒与中脑边缘多巴胺之间的直接关系，睡眠必须主要用于获得有效的清醒。

愉快的睡眠（即情绪性睡眠）只能存在于能够感觉到情绪的动物中。因此，尽管已经在无脊椎动物和原始脊椎动物中描述了类似睡眠的状态，但是睡眠愉悦感明显地标志着等温脊椎动物和冷血动物的睡眠之间的差异。