The influence of sleep and circadian rhythms on daytime functioning has long been noted. Psychologists and psychology-oriented researchers were among the first to investigate the role of sleep and circadian rhythms in motivation, emotion, cognition, and performance. At the dawn of modern sleep science, psychological fellow, F. B. Dresslar found that telegram tapping speed varied according to time of day providing the first clues to the role of circadian rhythms on performance (Dresslar 1892). Marie de Manacéïne was the first to experimentally demonstrate that sleep was critical to life and that the brain and behaviour were adversely impacted by sleep loss (de Manacéïne 1894) Patrick and Gilbert elaborated on the role of sleep on psychological functioning and were the first to experimentally study the impact of sleep loss in humans (Patrick and Gilbert 1896). The past century has seen an explosion of interest in understanding which aspects of sleep and circadian rhythms relate to which aspects of daytime psychological functioning.

Clinical sleep psychology is a branch of psychological science that aims to understand and harness psychological factors (i.e., affective, behavioral, cognitive, social, and environmental) that are involved in sleep and circadian disorders. Behavioral Sleep Medicine, a branch of sleep medicine, has a similar focus (https://www.behavioralsleep.org/). Collectively, these clinical branches of sleep psychology aim to effectively evaluate and treat sleep and circadian rhythm disorders to promote sleep health. Clinical sleep psychology has a long history, including early work in the areas of insomnia, behavioral treatment for pediatric sleep disorders, dream psychology, and hypnotic sleep (Jacobson 1934; McReynolds 1997; Mower and Mower 1938; Münsterberg 1915; Watson 1928). The past 100 years have seen considerable progress in identifying psychological factors involved in sleep and circadian rhythm disorders and their treatment, particularly in insomnia and pediatric sleep disorders. Barriers to advancement persist.

Sleep and circadian health disparities are persistent differences in sleep characteristics and circadian rhythms that have adverse outcomes and disproportionately affect vulnerable, marginalized, disadvantaged, minority, and minoritized populations. Inequalities in sleep duration, efficiency, timing, regularity, alertness, and quality are manifestations of and contributors to social injustice. Populations known to experience sleep and circadian health disparities include adolescents, indigenous people, refugees, asylum seekers, migrants, immigrants, racial and ethnic minorities, sexual and gender minorities, women, pregnant people, older adults, individuals with mental and physical illness or disabilities, some religious groups, the socioeconomically disadvantaged, the homeless, and those living in underserved rural areas (Billings et al 2021; Garbers et al 2023 Huber et al 2022; Moore et al 2023; Nandagiri et al 2023; Richter et al 2020; Roncoroni et al 2022; Schneeberger et al 2019; Yang et al 2023; Zarhin 2023). A nascent literature has begun to identify potential psycho-social-cultural determinants of sleep and circadian health disparities including social categorization, social identity, intersectionality, social power structures, socio-structural constraints, social, political, and economic context, discrimination, prejudice, and cultural sleep values (Billings et al 2021; Grandner et al 2016; Majeno et al 2023; Zarhin 2023). Many other factors are yet to be discovered. Research that informs our understanding of the factors that contribute to sleep and circadian health disparities can lead to tailored approaches for specific populations that are sensitive to sociocultural contexts, thereby improving health throughout the world (Etindele Sosso et al 2021).

Daily rhythms are primarily synchronized by the suprachiasmatic nucleus (SCN) in the hypothalamus, the body’s central circadian clock. The SCN aligns whole-body cellular, behavioral and phenomenological processes with the Earth’s 24-hour day/night rhythm. When disruptions to the SCN’s perceptual system occur (e.g., “jet-lag,” prolonged waking, chronic exposure to light at night or habit change as the result of becoming a parent), disrupted melatonin and glucocorticoid secretion can lead to widespread dysregulation of sleep and circadian cycling. The effects of circadian disruptions are often compounded by the homeostatic sleep drive, wherein sleep pressure accumulates with protracted wakefulness to affect mood, cognition, health and well-being.

Dreaming has always aroused our curiosity. Theories as to the cause and function of dreams have been described since the beginning of recorded history (George 2020). In the late 19th century, experimental psychologists and psychologically-minded researchers from other disciplines made important methodological contibutions, emptical observations, and conceptual developments to the study of dreams (e.g., Jastrow, 1888; Manacéïne, 1897; De Sanctis, 1899; Vold, 1897). At the end of the 19th century, Mary Whiton Calkins and her female students made pioneering advancements in the psychological science of dreams (Calkins 1893; Weed et al. 1896). Freud’s psychoanalytic theory soon overshadowed these groundbreaking empirical works as the interpretation of dream content and their presumed reflections of the unconscious mind became the focus. The detection of rapid eye movements during sleep in 1953 and the suggestion that dreams occurred exclusively during this newly defined sleep state electrified the field of dream research (Aserinsky and Kleitman 1953; Dement and Kleitman 1957). Although eye movements (Ladd, 1892), increased brain pulsations (Mosso, 1881), and electroencephalographic patterns (Loomis et al., 1937; Davis et al., 1938) had been previously argued to empirically correspond to dreaming, this discovery catalyzed the first “Meeting of Researchers in the Field of EEG and Dreams” at the University of Chicago in 1961 organized by psychologist Allan Rechtschaffen (Association for the Psychophysiological Study of Sleep Records). Renamed the Annual Meeting of the Association of the Psychophysiological Study of Sleep in subsequent years, these early meetings consisted principally of psychiatrists and psychologists, most of whom with interests in dream research. Among them, John Antrobus, Rosalind Cartwright, G. William Domhoff, David Foulkes, Donald R. Goodenough, Calvin S. Hall, Ernest Hartmann and Joe Kamiya, made valuable contributions to our understanding of dreaming through decades of psychological research (Antrobus, 1992; Domhoff and Kamyia, 1964; Ellman and Antrobus, 1991; Foulkes, 1966, 1985; Goodenough et al., 1965; Hall and Van de Castle, 1966; Hartmann, 2010). While David Foulkes tirelessly advocated for his vision of a descriptive and explanitory dream psychology, Rosalind Cartwright developed an applied vision for the field outlining over 100 dream-related questions that remain pertinent to sleep psychology (Cartwright 1977, 1978, 2010). With the rise of sleep medicine and the vicissitudes of funding, dream research drifted to the fringe of sleep research by the end of the 1980s (Foulkes 1996). Nevertheless, dreaming remains a central topic of sleep psychology, and many questions remain to be answered.

Understanding the intricate relationship between sleep, circadian rhythms and psychological development is a pressing question within the field of Sleep Psychology. This initiative calls upon researchers to submit manuscripts that contribute to answering the fundamental query, “What is the role of sleep and circadian rhythms in psychological development?” By delving into the intersection of sleep and psychological development, we aim to illuminate this critical connection and its implications for human well-being. Specifically, it is our hope that through addressing this question, we can better design and tailor interventions that aim to enhance psychological well-being across the lifespan.