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Sleep is one of the most complex and vital brain processes that aids in body restoration, memory consolidation, and energy recovery. Sleep is divided into two main stages: Non-Rapid Eye Movement (NREM) and Rapid Eye Movement (REM). Each of these stages has distinct effects on the brain and its functions.
1. Stages of Sleep
Sleep is generally composed of various cycles that include NREM and REM sleep stages.
• NREM Sleep: This includes three stages that progressively deepen. In the initial stages of NREM, brain activity is marked by slow and regular waves, which lead to a gradual reduction in awareness and information processing. In the third stage of NREM, known as “deep sleep” or slow-wave sleep (SWS), delta waves (low-frequency, high-amplitude brain waves) dominate. This stage is crucial for body repair and immune system strengthening.
• REM Sleep: After going through the NREM stages, the brain enters the REM stage. In this stage, brain activity resembles wakefulness while the person remains asleep. REM sleep is characterized by rapid eye movements, increased cortical activity, and the experience of dreams. This stage plays a key role in memory consolidation and the processing of emotional information.
2. Brain Regions Active During Sleep
• Thalamus: The thalamus serves as a central station for transmitting sensory information to the cortex. During the NREM stages, thalamic activity decreases, reducing sensory signals sent to the cortex, thereby decreasing awareness of the external environment. However, in the REM stage, the thalamus becomes active again and sends sensory signals to the cortex, though processing these signals is disrupted due to the inhibition of other sensory systems.
• Prefrontal Cortex: This region, responsible for decision-making, self-awareness, and thought control, shows reduced activity during REM sleep. This reduced activity in the prefrontal cortex may explain the illogical and imaginative nature of dreams.
• Hippocampus: The hippocampus plays an essential role in consolidating and processing long-term memories. During sleep, especially REM sleep, the hippocampus interacts with the cortex to organize and consolidate new information. This stage is vital for consolidating emotional and experiential memories.
• Default Mode Network (DMN): The DMN, which is involved in self-reflection and internal mental activities, remains active during REM sleep. This network plays a role in generating dream content and organizing thoughts during sleep.
3. Neurochemical Mechanisms of Sleep
• Melatonin: Melatonin is a hormone secreted by the pineal gland that regulates the sleep-wake cycle. This hormone increases in response to darkness and plays a primary role in initiating and maintaining sleep.
• GABA: GABA is an inhibitory neurotransmitter that is notably active during the NREM stages. Through inhibiting excitatory neurons, GABA reduces brain activity and helps the brain enter a state of relaxation and deep sleep.
• Acetylcholine and Norepinephrine: During REM sleep, acetylcholine levels increase, while norepinephrine levels decrease. Acetylcholine plays an essential role in activating brain functions during REM sleep and helps generate dreams. The decrease in norepinephrine helps maintain the sleep state, preventing the brain from entering wakefulness.
4. Differences Between Sleep and Anesthesia
• Awareness and Dreams: During sleep, especially in the REM stage, a person may dream and experience a kind of awareness of internal experiences. In contrast, in general anesthesia, there is no conscious experience or dream, as the areas responsible for creating awareness and memory are entirely suppressed.
• Brain Activity: In sleep, brain activity, especially during the REM stage, closely resembles wakefulness. In general anesthesia, however, key brain areas like the prefrontal cortex and thalamus are completely inactive, and no awareness or sensory processing occurs.
• Memory: Sleep, especially REM sleep, plays a crucial role in consolidating long-term memories. In general anesthesia, the hippocampus and other memory-related areas are deactivated, and no memory consolidation takes place.
References:
• Diekelmann, S., & Born, J. (2010). The memory function of sleep. *Nature Reviews Neuroscience*, 11(2), 114-126.
• Stickgold, R. (2005). Sleep-dependent memory consolidation. *Nature*, 437(7063), 1272-1278.
• Steriade, M. (2003). The corticothalamic system in sleep. *Frontiers in Bioscience*, 8, d878-d899.
• Hobson, J. A., & Pace-Schott, E. F. (2002). The cognitive neuroscience of sleep: Neuronal systems, consciousness, and learning. *Nature Reviews Neuroscience*, 3(9), 679-693.