Physiologists had proposed that some structure deep within the brain controlled mental wakefulness and alertness. The early symptom would be dominated by sleepiness or wakefulness. This suppresses daytime melatonin, enabling wakefulness. Sleep regulation refers to the control of when an organism transitions between sleep and wakefulness. Melanopsin may aid in controlling sleep cycles and wakefulness. Orexin (), also known as hypocretin, is a neuropeptide that regulates arousal, wakefulness, and appetite. CCK has been shown to interact with orexin neurons, which control appetite and wakefulness (sleep). They are the result of circadian rhythms cycling into a phase of wakefulness. It is neither wakefulness, dreaming, nor deep sleep. Aristotle describes sleep in "On Sleep and Wakefulness". An indication of cosmic consciousness is "ever present wakefulness" present even during sleep. The thalamus also plays an important role in regulating states of sleep and wakefulness. They may experience sleep-wake cycles, or be in a state of chronic wakefulness. HR antagonism leads to histamine release into the cerebrospinal fluid which promotes wakefulness. Replay has been found to occur during post-training quiet wakefulness as well as sleep.