What puts us to sleep? This question has bothered the mankind for thousands of years, but we still have no definite answer. After abandoning philosophical and religious explanations, science has adopted this question and started to examine it with experimental methods. Two early pioneers in this field, Dr. Ishimori from Japan and Drs. Pieron and Legendre from France developed the concept of hypnotoxin – a factor that accumulates during waking and puts animals and humans to sleep. They were able to show that, indeed, during deprivation of sleep, something accumulates in body – something that can be removed and will induce sleep in another individual. Later research has identified many substances that affect sleep. One of them is adenosine, which fulfils the criteria of a physiologic sleep factor.