We briefly describe the most significant findings obtained recently concerning the sleep-waking cycle in comparison to the studies conducted by Cajal on the same subject.This paper includes a short biographical sketch of Santiago Ramón y Cajal, with special emphasis on his importance within the framework of neuroscience. Cajal represents the decisive turning point in neurobiological studies, with the discovery of the synapse and his law of dynamic polarization. We conduct a short survey of the current knowledge about the phases of sleep and oneiric phenomena, based on their anatomo-physiological foundations. We present a summary of the history of the subject and analyze the contributions made by Cajal to this field, i.e. his study of the associative cortices, which are essential in memory processes and related to the mechanisms governing the sleep-waking cycle. For Cajal the fine anatomy of the thalamus must be considered in relation to the specificity of its connections an idea that is still completely valid today. He did not observe any projections of the thalamic reticular nucleus towards the cerebral cortex, a fact that has been corroborated using modern-day techniques. He spoke of the involvement of neuroglia in the attentional and sleep processes, which is so, although not quite in the way Cajal thought. He considered the production of dreams to be based on intimate neural mechanisms, which is still so. He also studied other brain structures related with the regulation of the sleep waking cycle, although avoiding any specific mention of the mechanisms controlling such a cycle. Furthermore, he conducted self-observation studies with a high degree of insight.Cajal studied the phenomena of attention and sleep in an objective manner and contributed a number of significant interpretations, some of which are now somewhat outdated while others are still wholly valid today.