Special Topic Issue: Biological Signals and Receptors 2000, Vol. 9, No. 6 Sleep is a physiological state of the brain during which many systems and/or networks are acting in totally diverse ways or, in some cases, in a very similar manner to the awake state. We have to understand how all the correlations that surely exist among the networks participate in sleep organization. We are just beginning to partially understand the influences that the brain receives which, acting in conjunction, will finally produce a sleeping brain. Different inputs may activate and/or deactivate neuronal networks at several CNS loci as well as change the central organization that, together with some unknown (intrinsic and extrinsic) factors, will provoke sleep. Sensory systems are part of this phenomenology. All the incoming information from outside and from our own body modulates the CNS activity that occurs during the state of wakefulness and also - continuously - during both sleep phases. Four of the works presented here are related to sensory signals. Temperature, bright light, auditory and visual stimulation as well as, modulation of sensory influences by the amygdala. The other two approaches consider ‘wet‘ signals as adenosine-mediated sleepiness and the melatonin effect on human sleep. Basic and clinical sleep researchers will become aware of the new approach to this still rather unknown phenomenon: sleep.