The question about the function ofsleep remains one of the major challenges scientists are faced with. Wherein lies the fascination with sleep? I am convinced that it is the necessity for sleep. No one has failed to experience the overpowering urge to fall asleep after a disturbed night's sleep or after sleep was curtailed or deprived, especially when our daily activities impose restrictions on motor activity. The demand ofour body and brain to sleep challenges our understanding ofwhy this is the case, and which are the benefits ofa night ofprofound sleep. Also in animals prolongation of waking consistently increases their attempts to fall asleep. It has been stated that sleep is more necessary to animals than even food! The need for sleep and some insight into the consequences of the preceding daily waking activities on subsequent sleep was wonderfully formulated by Shakespeare in Othello: Not poppy nor mandragora, Nor all the drowsy syrups of the world, Shall ever medicine thee to that sweet sleep Which thou owed'st yesterday It is interesting that the most powerful single intervention which invariably influences sleep in a positive and predictable manner is the prolongation of waking. The activities which people or animals engage in during the wakefulness episode are secondary in the magnitude of their effects on sleep.