Ninety-five percent or more of dreams are populated by the dreamer who interacts with two to four other characters, most of whom can be recognized as familiar characters in the dreamer’s immediate social network. Friendly interactions (typically verbal conversations) are found in about 40 percent of dreams, while aggressive social interactions occur in about 45 percent of dreams. In addition, mind-reading or inferring the mental states of others, particularly those characters the dreamer interacts with, occurs in over 80 percent of dreams. Finally, people who are most important in the dreamer’s waking network regularly appear in that dreamer’s dreams. Thus, existing data from dream content studies is certainly consistent with SST.

There are three basic brain states: waking, REM, and NREM sleep. What determines or creates and maintains each of these three states is a differing mixture or profile of brainstem-generated neurotransmitter (aminergic and cholinergic modulation) activity levels as well as differing forebrain activation and deactivation patterns, which were discussed in previous chapters. The three different brain activity profiles that give rise to the three different brain states must be thought of as probabilistic profiles. Each brain state’s profile can be fully engaged or only partially engaged. Most importantly for understanding the experiences associated with parasomnias, the transitions between the brain states can also be complete or only partial. When one state ends another state begins if the transition between states is complete. But because the mechanisms that control brain states are probabilistic, transitions between states are virtually never entirely complete. When transitions between states are partial we get a hybrid brain state, for example, a mixture of REM and waking or a mixture of NREM and waking or REM with NREM. When these hybrid states occur we get the classic parasomnias.

Sleep in the form of regularly occurring periods of quiescence and some amount of sleep rebound can be found in even the simplest of organisms from earthworms and fruit flies to nonhuman primates and human beings. We do not see evidence, however, of the emergence of distinct sleep states until we come to the reptiles. Birds and aquatic mammals also evidence distinct sleep states including the phenomenon of unihemispheric sleep, which allows these animals to sleep while flying or swimming. REM may only occur bihemispherically. The presence of high voltage slow waves as well as REM-like brain activation patterns in reptiles, birds, and mammals suggests that the biphasic, REM, and NREM sleep phases we find in humans is a very ancient adaptation indeed and that its benefits outweigh the risks associated with quiescence and reduced responsiveness to the environment.

The time it takes to fall asleep (latency) declines until midlife and then remains about the same into old age. Time spent awake after initial sleep onset (WASO) declines across the lifespan but its proportion of total sleep period increases. That is, people tend to have a greater number of awakenings as they age. REM percentages decline with age but the proportion of total sleep spent in REM remains about the same. The same is the case with N2 stage light sleep and N1 transitional sleep; these proportions remain about the same or slightly increase as people age. Finally, N3 slow wave sleep undergoes a steady decline with age until it almost completely disappears in old age. Throughout the lifespan sleep evidences intimate and possibly bidirectional causal associations with socio-emotional attachment processes between child and parent during the developmental phase and then between sexual/romantic and close friends during the adult phase. These relationships between sleep processes and attachment processes once more underline the social nature of sleep.