A new theoretical approach to the functional meaning of sleep and dreaming in humans based on the maintenance of 'predictive psychic homeostasis'

Abstract

Different theories have been put forward during the last decade to explain the functional meaning of sleep and dreaming in humans. In the present paper, a new theory is presented which, while taking advantage of these earlier theories, introduces the following new and original aspects: • Circadian rhythms relevant to various organs of the body affect the reciprocal interactions which operate to maintain constancy of the internal milieu and thereby also affect the sleep/wakefulness cycle. Particular attention is given to the constancy of natraemia and osmolarity and to the permissive role that the evolution of renal function has had for the evolution of the central nervous system and its integrative actions. • The resetting of neuro-endocrine controls at the onset of wakefulness leads to the acquisition of new information and its integration within previously stored memories. This point is dealt with in relation to Moore-Ede's proposal for the existence of a 'predictive homeostasis'. • The concept of 'psychic homeostasis' is introduced and is considered as one of the most important states since it is aimed at the well-being, or eudemonia, of the human psyche. Sleep and dreaming in humans are discussed as important functions for the maintenance of a newly proposed composite state: that of 'predictive psychic homeostasis'. On the basis of these assumptions, and in accordance with the available neurobiological data, the present paper puts forward the novel hypothesis that sleep and dreaming play important functions in humans by compensating for psychic allostatic overloads. Hence, both consolatory dreams and disturbing nightmares can be part of the vis medicatrix naturae, the natural healing power, in this case, the state of eudemonia.

Keywords: circadian rhythms of peripheral organs; evolutionary tinkering; homeostasis of internal milieu; predictive psychic homeostasis; sleep and dream theories.

Figure 1.

Upper Panel: Schematic representation of the concept of the Internal Milieu (that is, the medium in contact with the cells of a multi-cellular organism), and of the concept of homeostasis. Lower Panel: box representation of some main logical steps involved in the homeostatic process and of the relevance of feed-back signals, that is, of signals which inform the control systems of the 'load error' (discrepancy between the observed value of a physico-chemico parameter and its optimal value) which needs to be compensated for in order to restore optimal parameters. This can be accomplished by changes in the performance of relevant organ systems.

Figure 2.

Schematic representation of the main functional connections between the composition of the internal environment and the integrative actions of the complex cellular networks in the brain, as they relate to sleep and dreaming.

Figure 3.

Schematic representation of the multiple functional interconnections between heart and brain, according to Thayer and Lane. Special emphasis is given to the action of cardiac activity on brain integrative functions. It is worth noting that vagus nerve stimulation is employed for the treatment of neuropsychiatric disorders.^, For further details, see text.

Figure 4.

Schematic representation of some aspects of the present hypothesis of ‘Psychic Predictive Homeostasis’ (PPH). The main assumption is that, together, sleep and dreaming play a role in the maintenance of psychic homeostasis in a predictive mode. Thus, sleep and dreaming are aimed not only at correcting the psychic allostatic loads suffered by the subject during the daytime, but also at preparing him for the likely psychic loads of the next day. For further details, see text.