Why Do We Need to Sleep and Dream?
An ancient question with modern answers—and enduring mysteries
The function of sleep and dreams has been studied for millennia. The oldest texts on these topics come from Ancient Egypt. Today, despite growing experimental and clinical research, we remain far from a coherent, complete picture. We're beginning to understand why we need sleep—its importance is becoming clear, but dream remains largely a mystery.
Ancient Egypt: The Spiritual Approach
In ancient times, Egypt possessed the most advanced medicine and surgery. It was the place to be for medical services (For more details about the medical professions and statuses in Ancient Egypt, see this chapter). The Egyptian approach to sleep was highly spiritual, reflecting their conception of humans as both material and spiritual beings. Physicians were often priests—medicine and spirituality were inseparable.
Medical papyri (see here the lists of the ten main medical papyri officially discovered) include diverse remedies for sleep disorders and snoring. Hypnagogic states were induced using anaesthetics and hypnosis (trance-like states), and the visions reported by patients served as diagnostic tools to determine cures.
The Qenherkhepshef Papyrus, now owned by the British Museum, is the oldest text about dream interpretation. Dreams were considered a state of consciousness allowing reception of messages from gods or spiritual beings generally. This conception, though not recognised by the scientific community, has survived to modern days.
Despite many speculations, the function and underlying mechanisms of dreams are still not well understood.
Qenherkhepshef Papyrus (also known as Papyrus Chester Beatty III. Sheet 1) recto, columns 1-3: Hieratic dream book
Source: British Museum. More details here.
Neuroscience of Sleep: What We Know
Sleep is characterised by absence of behavioural response and apparent extinction of consciousness. This led to the early assumption that sleep serves the nervous system—that the brain needs sleep.
The first scientific sleep studies, enabled by electroencephalography (EEG), focused on describing brain activity during sleep. Brain activity cycles through stages repeated throughout the night, divided into three main phases:
Light sleep
Deep sleep (also called Slow Wave Sleep, SWS)
Rapid Eye Movement sleep (REM)
Across the night, deep sleep stages shorten in successive cycles whilst REM stages lengthen.
Brain activity cycles through stages repeated throughout the night, divided into three main phases
Brain Activity Across Sleep Stages
Deep Sleep: Metabolic activity reduces by approximately 40%. Brain activity reaches its lowest level.
REM Sleep: Metabolic activity similar to wakefulness. EEG activity resembles the waking state.
The Dream Misconception: REM ≠ Dream
Dreams were initially associated with REM sleep following Aserinsky and Kleitman's discovery of this sleep stage in 1953. For decades, dream and REM sleep were treated as equivalent—as if brain activity during REM directly caused dream experience.
This assumption was false.
This equivalence was more assumption than fact—typical of contemporary neuroscience's tendency to presuppose equivalence between neuronal activity and conscious mental activity.
Studies (Foulkes 1962; Oudiette et al. 2012; Carr and Solomonova 2019) provide clear evidence: dreams are also reported outside REM sleep.
Neurophysiological activity may impact the nature of dream contents, memorisation, or recall—but REM activity does not equal dreaming, and dreaming is not confined to REM.
Dreams and Micro-Awakenings: The Emerging Picture
Recent research reveals that dreams are more likely to occur during micro-awakening phases—brief moments when the brain shifts toward wakefulness without fully waking the sleeper.
These micro-awakenings represent transient increases in cortical activation. They're not full awakenings—sleepers remain asleep—but brain activity briefly rises above the deep sleep baseline.
Implications: This suggests dreaming may require a threshold level of cortical activity—enough to support conscious experience but not enough to fully wake. Dreams might occupy a liminal state between sleep and waking consciousness, neither fully one nor the other.
This complicates the picture further. If dreams occur during micro-awakenings rather than deep REM, what is the relationship between sleep architecture and conscious experience? Are dreams a byproduct of incomplete sleep, or do micro-awakenings serve some function that requires dream-like mentation?
We don't know. The physiology underlying dreaming remains poorly understood—and this lack of understanding continues to feed spiritual conceptions of dream function.
Sleep Is for the Whole Body, Not Just the Brain
Sleep is triggered by mechanisms in the nervous system, but serves wider physiological functions. Sleep is needed for the whole body, for all cells.
Not only quantity matters—regularity and duration of each stage are critical. Different stages appear to serve different functions, and their disruption impacts health differently.
Consequences of Sleep Deprivation and Fragmentation
Endocrine, metabolic, and immune dysregulation
Disrupted hormone regulation
Impaired glucose metabolism, increased diabetes risk
Weakened immune function
Brain function impacts
Mood dysregulation and emotional instability
Lowered pain sensation threshold (increased pain sensitivity)
Altered reward and risk perception
Impaired attention and memory performance during wakefulness
Circadian Disruption and Cancer
Recent research (Sulli et al. 2019) demonstrates that disruption of the circadian clock—the 24-hour internal cycle in our brain regulating alertness and sleepiness in response to light—is associated with cancer.
Working night shifts or late shifts increases cancer risk. The mechanism appears to involve circadian gene expression regulating cell division, DNA repair, and apoptosis. When these rhythms are disrupted, cellular processes lose coordination.
Recent research (Sulli et al. 2019) demonstrates that disruption of the circadian clock—the 24-hour internal cycle in our brain regulating alertness and sleepiness in response to light—is associated with cancer.
Integrating circadian biology into cancer research may improve quality of life, enable cancer prevention, and make treatments more effective by timing interventions to match the body's natural rhythms.
What We Know, What Remains Mystery
Sleep: Increasingly understood
The importance and benefits of sleep for physiological health—including mental health—are increasingly studied, recognised, and established. Sleep serves metabolic regulation, immune function, cellular repair, memory consolidation, emotional processing, and countless other functions essential for survival.
We understand that we need sleep and increasingly understand why.
Dreams: Still mysterious
The function of dreams—once thought equivalent to REM sleep—remains unclear. Dreams occur outside REM. They appear more likely during micro-awakenings. The physiology underlying dreaming is poorly understood.
Do dreams serve a function? Are they epiphenomena—byproducts of neural activity without adaptive purpose? Do they process emotions, consolidate memories, simulate threats, or serve some other cognitive function? Or do they, as Ancient Egyptians believed, represent something genuinely distinct from ordinary waking consciousness—a state with its own phenomenology that cannot be reduced to neurophysiological mechanisms?
Contemporary neuroscience has long presumed that neural activity fully explains conscious experience. But near-death experience research has provided solid, unambiguous evidence against this presumption—demonstrating that conscious mental activity can occur when neuronal activity is minimal or totally absent. The persistent mystery of dreams—their resistance to neat functional explanations, their occurrence outside predicted states, their phenomenological richness—is consistent with this broader evidence that consciousness and brain activity are not perfectly equivalent.
My sincere opinion about this is that the Ancient Egyptians were likely correct to approach dreams as something requiring interpretation beyond purely physiological mechanisms. Not necessarily because the possibility of communication from ancestors, guides, or other entities, but because subjective conscious experience—dreaming included—does not reduce cleanly to brain states. If consciousness (or its substrates, whatever it is) survives brain death and is capable of perception, memory, and emotion, it main remains a physical phenomenon (a magnetic field, a quantum field are physical objects) able to interact with the brain. The conscious substrate might have its own independent activity, conducting its own business at night in parallel with the brain conducting its own processes.
This lack of understanding is not failure. It's honesty about the limits of current knowledge—and acknowledgment of evidence that challenges reductionist assumptions. Sleep, we increasingly understand. Dreams remain, for now, a mystery.
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Key references:
Aserinsky, E., & Kleitman, N. (1953). Regularly occurring periods of eye motility, and concomitant phenomena, during sleep. Science, 118(3062), 273-274.
Foulkes, D. (1962). Dream reports from different stages of sleep. Journal of Abnormal and Social Psychology, 65(1), 14-25.
Oudiette, D., et al. (2012). Dreaming without REM sleep. Consciousness and Cognition, 21(3), 1129-1140.
Carr, M., & Solomonova, E. (2019). Dream recall and content alongside polysomnographic and home sleep monitoring. In Sleep and Affect (pp. 325-350). Academic Press.
Sulli, G., et al. (2019). Pharmacological activation of REV-ERBs is lethal in cancer and oncogene-induced senescence. Nature, 553(7688), 351-355.
