The mind in the midnight hour

Priasha Dutta, Amity University Kolkata

Stages of sleep:

Various research studies have elucidated the various aspects of the mind in the midnight hour. While we sleep and have dreams, our brain is directly involved in regulating our sleep cycles and monitoring sleep and dream quality. Sleep has two stages:

(i) REM (rapid eye movement)

(ii) non-REM, which is further divided into three stages.

Each is linked to certain brain waves and neuronal activity. One cycles through all stages of non-REM and REM sleep many times during the night, with increasingly longer and deeper REM periods occurring towards morning.

The three sub-stages of NREM (non-REM) sleep are:

Stage 1 NREM sleep: It is the transition phase from being awake to sleep. During this short period of lighter sleep, the heartbeat, breathing, and eye movements slow down and the muscles relax. The brain waves begin to slow down as compared to their daytime wakefulness patterns.

Stage 2 NREM sleep is a period of sleep before entering a deeper state. The body temperature and eye movements decrease. The heartbeat and breathing rates slow, and muscles relax further. Brain wave activity slows but includes short bursts of electrical activity. Stage 2 is the majority of our sleep cycle.

Stage 3 NREM sleep is the period of deep sleep and occurs in longer periods during the first half of the night. It is difficult to wake someone in this stage and the muscles are completely relaxed. Heartbeat and breathing slow down the most. Brain waves become even slower.

REM sleep first occurs after about 90 minutes of falling asleep. The eyes move sideways rapidly behind closed eyelids. The mixed frequency brain wave activity resembles that seen in wakefulness. Breathing becomes faster and irregular. The heart rate and blood pressure increase to near waking levels. REM sleep becomes of lesser time as we age.

What is a dream?

A dream is a succession of emotions, sensations, ideas, and images that usually occur involuntarily in the mind in the midnight hour. They occur in the typical two hours of REM during eight-hour night sleep. As the night progresses, they tend to last longer. Sometimes they can occur in other phases, but those are easily forgotten. The length of a dream can vary and people are more likely to recall them if they are awakened during the REM stage. An individual has around three to five dreams per night. Dream patterns can also vary with different neurological disorders.

During dreams, reflective consciousness can be recovered, what is known as lucid dreaming (LD). In LD the person is aware that he/she is dreaming and can direct the occurring events. LD is more frequent in children than in teenagers and adults. Compared to non-lucid REM sleep, LD REM sleep is related to local frontal lobe EEG changes with increased brain coherence. LD is linked to increased physiological activation, and autonomic nervous system arousal is also elevated. LD is related to amplified cortical activation, and the prefrontal cortex is more active than during regular dreaming.

The areas of the brain involved in regulation of the activities of the mind in the midnight hour:

• The hypothalamus contains groups of nerve cells that affect sleep and arousal.
• The suprachiasmatic nucleus (SCN) are clusters of thousands of cells within the hypothalamus that receive information about light exposure directly from the eyes and control the behavioral rhythm. People with damaged suprachiasmatic nucleus tend to sleep erratically almost throughout the day since they are unable to match their circadian rhythms with the light-dark cycle.
• The brain stem communicates with the hypothalamus to control the transitions between wake and sleep. Clusters of sleep-promoting neurons in several parts of the brain become more active as we go to sleep. Sleep-promoting cells within the hypothalamus and the brain stem produce GABA (gamma-aminobutyric acid), whose activity reduces the functioning of arousal centers in the hypothalamus and brain stem. GABA is strongly associated with sleep, sedation, and muscle relaxation. GABAergic processes are mainly responsible for REM sleep. Norepinephrine and orexin (also known as hypocretin) keep some parts active while we are awake. Some other neurotransmitters that influence sleep and wakefulness patterns are histamine, acetylcholine, adrenaline, serotonin, and cortisol.

Stimulation of the cortical and hippocampal areas, with signals by medulla and pons, relax the muscles and prevent the individual from acting out in dreams. A poorly understood neuronal network is linked with it. However, multiple nuclei and neurotransmission systems that are concerned with sleep muscle paralysis are known. In wake and REM sleep, the subcortical cholinergic tone is active and has a prolonged depolarization in cortical neurons with EEG activation.

The reticular activating system (RAS) is a network of neurons (nerve cells) that are located in or near the brain stem. Its direct extension to the cortex regulates the activation of awake.

The thalamus transmits information from the sense organs to the cerebral cortex. During most stages of sleep, the thalamus becomes inactive but during REM sleep, the thalamus is active, sending the cortex sounds, images, and other sensations that compose dreams.

The pineal gland is located within the brain’s two hemispheres, receives signals from the SCN, and increases the production of melatonin, which induces sleep once the lights go out. Blind people cannot coordinate their natural wake-sleep cycle using natural light, and thus stabilize their sleep patterns by taking small amounts of melatonin at the same time every day. Studies suggest that peaks and valleys of melatonin over time are vital for matching the circadian rhythm of the body to the external cycle of light and darkness.

The basal forebrain, located near the front and bottom of the brain, also induces sleep and wakefulness, while a certain region of the midbrain acts as an arousal system. The release of adenosine from cells in the basal forebrain supports the sleep drive. Caffeine counters sleepiness by blocking adenosine receptors.

The limbic system in the mid-brain includes the amygdala that is also involved in the activities of the mind in the midnight hour. For instance, it deals with emotions in waking and dreaming, and it is typically associated with fear. Experiences of nightmares suggest increased amygdala activation during REM sleep.

Electroencephalogram (EEG) studies of dreaming:

Specific dream characteristics suggest the activation of particular brain regions during sleep. fMRI scans and EEG tests are tools to monitor neural mechanisms. The whole brain is active during dreams, which means that the sleep is not so deep in the REM (sometimes in non-REM sleep too). The activity levels are like when one is awake, so one has deep dreams in REM sleep. In both non-REM and REM sleep reporting of dream experience, the parieto-occipital hot-zone has high involvement. In cases of dreaming, the local decrease in low-frequency EEG activity occurs. This suggests that there can be conscious experiences in sleep.

Since studies have indicated how dreams burst into similar parts of the brain that involve reality in waking states, dreams are known as a form of consciousness that occurs during sleep. Forgotten or absent dream experiences carry a different EEG mark. Neuroimaging data show that the high visual content of dreams is related to the widespread activity along occipital–temporal visual regions. Several regions are considerably hypoactive during REM sleep compared to wakefulness, especially the dorsolateral prefrontal cortex (DLPFC), orbitofrontal and inferior parietal cortex, posterior cingulate gyrus, and the precuneus. Dreams related to waking-life experiences are linked with frontal theta activity, which proposes that emotional memory processing can occur in the REM stage.

The neurobiology of LD is still uncharacterized due to the varied EEG results and limited neuroimaging data. However, primary results suggest that prefrontal and parietal regions are highly involved. Cortical areas activated during LD overlap with brain regions impaired in psychotic patients like schizophrenia, especially frontoparietal regions. This is probably because both have subjective experiences. Thus, the initial evidence points out that regions of the anterior prefrontal, temporal and parietal cortex are compromised in LD; the same brain regions are involved with metacognitive processes during the waking state. LD is further linked to the reactivation of areas usually deactivated during REM sleep. This explains the recovery of reflective cognitive capabilities that are the prime characteristic of LD. Such findings may correspond to restored reflective consciousness. Thus, the work of the mind in the midnight hour has been understood to some extent. However, more research and experimental findings will provide more intriguing data in the future.

Also read: ABCG2 contributes to multidrug resistance of cancer cells

References:

1. Mistérios, R., Sonhos, N.D., & Gomes, M. (2020). Unveiling sleep mysteries: neurobiology of dreaming.Carlo Cipolli, Luigi De Gennaro (2020)- “Neurobiology of Dreams” https://doi.org/10.1007/978-3-030-54359-4_5
2. Baird, B., Mota-Rolim, S. A., & Dresler, M. (2019). The cognitive neuroscience of lucid dreaming. Neuroscience and biobehavioral reviews, 100, 305–323. https://doi.org/10.1016/j.neubiorev.2019.03.008

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