1 | INTRODUCTION

When mentioned on its own, sleep is often portrayed as the opposite of waking and of consciousness (Searle, 2000; Tononi, 2008). This would seem to imply that dreamless sleep hovers on the borders of consciousness science and philosophy of mind but falls outside of their purview.

This situation is symptomatic of the often implicit idea that sleep is both all-too familiar and interesting mostly for what it lacks.The obvious candidates for such a negative characterization include conscious experience, overt behavior, and ability to react to external stimuli. But new findings and theoretical developments suggest that this traditional view is misleading: we think we know what sleep is, but in a number of important ways, we are only just beginning to find out. Consciousness science is increasingly engaging with these findings, but philosophy has been slow to follow suit.

[in studying sleep] we will gain important insights on the variability, richness, and subtleties of sleep-related experience; we will likely also have to revise our taxonomy of mental states, including our understanding of sleep and wakefulness themselves.

I begin by reconstructing the traditional view of sleep and how it has already changed in response to scientific findings. I propose that so-called #simulation_views of #dreaming can help the field move forward. They promise a phenomenological framework in which dreaming is defined independently of #sleep stages while also forming a point of departure for identifying kinds of sleep experience that can be described as #dreamless.

Together with advances in the #neuroscience_of_sleep, including #sleep_staging and #sleep_disorders, this framework can guide a novel conception of sleep and sleep staging and lead us toward a theory that unites the subjective and objective sides of sleep. To show how, I consider empirical evidence on #sleep_onset, #state_dissociations, and #local_sleep. I suggest radical revisions to the traditional concept of sleep, wakefulness, and conventional sleep staging are needed and identify questions for future research.

2 | THE TRADITIONAL VIEW OF SLEEP

Sleep has traditionally been thought of as a naturally and regularly occurring period of passivity and rest that affects the entire organism, including behavior and responsiveness to external stimuli as well as the occurrence and contents of subjective experience.

Defined by the:

• absence of behavior,
• absence of perception,
• absence of (with the exception of dreams) consciousness,
• Also sleep is also commonly regarded as a uniform state and the opposite of wakefulness.

A bird's eye view on the history of sleep and dream research to new findings

Despite [recent] refinements, the basic ingredients of the traditional view have largely remained intact. Moreover, while sleep and dream research are often considered as separate fields (Kroker, 2007), this division is artificial, and how sleep relates to consciousness (including dreams) is and always has been a central question for the definition and scientific study of sleep.

Aristotle (1990) characterized sleep through a global loss of perception and promoted a dichotomy between sleep and wakefulness in which sleep is quite simply the absence of wakefulness. *Later this dichotomy was extended to cover the contrast between sleep and consciousness, which in line with behaviorism was equated with wakefulness (Malcolm, 1959).

this negative view of sleep discouraged systematic research, conveying the impression that in sleep, there was nothing interesting to observe (Kroker, 2007). It finds its starkest expression in the description, again of ancient origin, of sleep as the little brother of death.

Sleep science has added complexity to the definition of sleep but largely preserved the main ingredients of the traditional view. Behaviorally, sleep is characterized by:

• immobility,
• relaxed posture,
• and reduced responsiveness (Peigneux et al., 2001; Siegel, 2009).
• easily reversible
• and homeostatically regulated ***meaning that sleep loss results in increased duration, depth, or consolidation of sleep (Cirelli & Tononi, 2008).

#animal_Sleep -> the functions of sleep and its ubiquity in all animals remain controversial (Cirelli & Tononi, 2008; Siegel, 2008)

Some changes

The behavioral criterion of unresponsiveness [along with] brain-based criteria [helped] identifying different sleep stages. [Also], with the discovery of rapid eye movement ( #REM_sleep ) (Dement & Kleitman, 1957), sleep was seen to have a complex architecture in which the stages of #non-REM ( #NREM ) Stages of #non-REM stretch from sleep onset to deep sleep, are followed by REM sleep (Pace-Schott, 2009).

#NREM sleep is marked by synchronized slow-wave activity, whereas #REM sleep is marked by desynchronized fast activity that is nearly indistinguishable from waking - this is knowin by #EEG.

Muscle tone decreases in #NREM sleep as compared to wakefulness and reaches its low-point in #REM sleep

This state of near-paralysis #NRM contrasts with rapid eye movement activity. It also contrasts with reports of rich subjective experiences, which are much more likely to follow awakenings from REM than from NREM sleep.

Throughout the night, we follow a standard trajectory through the stages of NREM sleep, followed by REM sleep, a return to NREM sleep, and so on; sleep is also interspersed with numerous awakenings, often too short to be recalled.

Changes in sleep are regulated by the ascending reticular activating system.

The reticular activating system's fundamental role is regulating arousal and sleep−wake transitions. The ascending reticular activating system projects to the intralaminar nuclei of the thalami, which projects diffusely to the cerebral cortex.

Each sleep cycle takes about 90 min, but with increasing sleep duration, the deeper stages of NREM sleep disappear and give way to longer REM periods. The longest and most complex dreams occur in the morning, close to awakening.

The discovery of REM sleep in the 1950s marked the beginning of both sleep and dream science [and optimistycally], [this moved] early dream researchers to regard the physiological characteristics of REM sleep as objective markers of the occurrence and duration of dreaming (Dement & Kleitman, 1957). [which in turn, led to] to the view of NREM sleep as unconscious, dreamless sleep.

These findings initiated a profound shift in the concept of sleep: NREM sleep inherited the negative definition that had previously been applied to sleep in general. By contrast, to make sense of the occurrence of conscious experience in sleep, dreaming was confined to REM sleep and assigned a category of its own.

REM sleep was “neither sleeping nor waking. It was obviously a third state of the brain, as different from sleep as sleep is from wakefulness” (Jouvet 2000, p. 5). This notion of waking, REM, and NREM sleep as the three cardinal states of the brain and mind continues to be widespread.

[this is] for example, central to the influential #AIM_model (Hobson, Pace-Schott, & Stickgold, 2000) [which] attributes transitions from waking via NREM to REM sleep, [and] changes in conscious experience, to concerted changes in the overall level of brain activation (A), information processing/input source (I), and neuromodulation (M) . The AIM model preserves the original idea of waking, REM and NREM sleep as global states, assuming isomorphism on the phenomenological, behavioral, and neuronal levels.

The idea that sleep, wakefulness, and sleep-stages are multilevel, whole-brain phenomena that can be unambiguously categorized and are mutually exclusive is popular to this day (Siclari & Tononi, 2017). The picture has become more fine-grained, differentiating three states where previously there were thought to be just two, while preserving the main assumptions of the traditional view of sleep.

The traditional sleep–wake dichotomy also resurfaced in consciousness science in the form of a dichotomy between consciousness, associated with both waking and dreamful sleep, and unconscious, dreamless sleep. In this context, consciousness refers to the global phenomenon of being conscious at all, not to particular contents of consciousness (such as tasting pistachio ice cream or seeing a toddler smile). Used in the former sense, consciousness is often operationally defined as that which disappears in deep, dreamless sleep and reappears in waking and dreaming (Searle, 2000; Tononi, 2008). A similar idea is found in work on #global_states, #background_states, or #modes_of_consciousness, where REM sleep/dreaming is contrasted with presumably unconscious NREM sleep (for details and critical discussion, see Bayne, Hohwy, & Owen, 2016).

the contrast between dreamful and dreamless (presumably unconscious) sleep has also been proposed as a research model of consciousness (Churchland, 1988; Revonsuo, 2006) and led to specific experiments probing the neural correlates of consciousness (Siclari et al., 2017; Tononi & Massimini, 2008).

The aspect of the traditional view that has undergone the most radical transformation is the characterization of sleep as passive and inactive . On the whole, findings on sleep architecture and sleep homeostasis (Cirelli & Tononi, 2008) have shown the progression of sleep stages to be actively orchestrated. Yet the view of sleep as passive rest was partially redeemed through the distinction between REM sleep/dreaming on one hand and unconscious NREM sleep on the other hand, alongside the view that sleep proper is confined to NREM sleep (Jouvet, 2000).

Passivity, in relation to NREM sleep, refers to the presumed absence of mental activity and conscious experience. By contrast, REM sleep is highly active on measures of brain activity and subjective experience. Areas associated with visual and motor experience and emotions are even more active in REM sleep than in waking, which in turn fits in well with the visual, action-packed, dynamic, and emotional character of many dreams (Desseilles et al., 2011). ***This stands in stark contrast to the fact that external stimulus processing and muscle tone reach their low point in REM sleep (this is also often called the sensory input and motor output blockade; Hobson et al. 2000);

in this sense, REM sleep is deeper and more behaviorally passive than NREM sleep. This contrast between outward passivity and rich subjective experience is reflected in the occasional designation of REM sleep as #paradoxical_sleep (Jouvet, 2000)

In sum, while the traditional view of sleep has been updated in response to scientific discoveries, many of its key assumptions have been preserved. Recurring themes include the tendency toward drawing sharp, mutually exclusive distinctions between sleep and wakefulness as well as between sleep stages; the tendency to regard them as whole-brain phenomena and global states in which changes in behavior and neural processing are aligned with changes in experience; and the idea that sleep, with the exception of dreaming, is uniformly unconscious. developments in the fields of sleep and dreaming cast doubt on these assumptions, making even the updated view of sleep and sleep staging seem insufficient.