This article focuses on the function of human sleep architecture and, where it adjoins, on the ultradian rhythm of NREM and REM cycles. In healthy adult human sleep, NREM and REM sleep succeed each other in 90-110 min intervals. This ultradian pattern of NREM/REM succession is cyclical. Sleep architecture relates to the shifting between sleep stages of varying sleep intensity and arousal thresholds. It has been found to follow a typical but non-cyclical pattern throughout the night. In the first third of the night, light sleep alternates predominantly with slow wave sleep (SWS). As sleep progresses, the amount of SWS typically decreases and REM sleep increases. In the last third of the night, light sleep almost exclusively alternates with REM sleep. The author postulates that one function of the non-cyclical succession of sleep stages is the erection and maintenance of a protective field around the sleeper. A protective field is to be understood as an area of relative safety, minimizing the chances for an intruder to enter the field without detection. The frequent shifts between sleep phases with low and high arousal thresholds allow a periodic screening of the sleep environment for danger signals. The relevant literature on sleep architecture, ultradian rhythms and its determinants is reviewed and the implications of the concept of protective fields are discussed.