Exploring Sleep: Heightened Senses Or Senselessness?

Sleep is not a passive activity, but a period during which the brain engages in several activities necessary for life. During sleep, the brain cycles through rapid-eye movement (REM) sleep and non-rapid eye movement (NREM) sleep. While the exact nature of each stage of sleep is still up for debate, it is known that sleep plays a critical role in solidifying and compiling memories. Lack of sleep can affect brain plasticity, or the brain's ability to process new information and input, which can slow down its interpretation of sensory input. This can affect all five senses, including vision and hearing. For example, in a sleep-deprived state, you may notice that you react to sounds more slowly. However, the senses are still operating while you sleep, and external stimuli such as light or sound can wake you up.

Your senses are dulled, but not turned off

Sleep is not a passive activity, but a period during which the brain engages in various activities necessary for life. Throughout sleep, the brain cycles repeatedly through two types of sleep: rapid-eye movement (REM) sleep and non-REM sleep. During non-REM sleep, the body and brain slow down, and the brain shows a pattern of pulses of activity believed to help prevent unwanted awakenings. During REM sleep, brain activity shoots back up to levels similar to when you're awake, which explains why it is associated with intense dreams.

While you sleep, your senses are not turned off, but they are dulled. For example, your eyes are still taking in visual information, even when your eyelids are closed. If someone turns on the lights, there's a good chance you'll wake up. The same is true of your sense of touch, hearing, and smell. However, the brain's ability to interpret input slows down during sleep, affecting all of the senses. In a sleep-deprived state, you might notice that you react to sounds more slowly, and your mental clarity may be impacted.

The thalamus, located in the brain, sends and receives information from the senses to the cerebral cortex, which is responsible for interpreting and processing memory. During most stages of sleep, the thalamus becomes quiet, allowing you to tune out external stimuli. However, during REM sleep, the thalamus is active, sending the cortex images, sounds, and other sensations that fill our dreams.

While the senses remain active during sleep, the brain's ability to process and interpret sensory input is diminished. This is because sleep deprivation negatively affects brain plasticity, or the brain's ability to adapt to and process new information. As a result, the interpretation of sensory input slows down, impacting all of the senses. Therefore, while the senses are not turned off during sleep, they are certainly dulled, and the brain's ability to process sensory information is reduced.

Your eyes still take in visual information

While you sleep, your eyes continue to take in visual information, even when your eyelids are closed. This is why you can be awakened by a light being turned on, or daylight starting to stream in through the window.

During sleep, your brain cycles repeatedly through two different types of sleep: rapid-eye movement (REM) sleep and non-rapid eye movement (NREM) sleep. The first part of the cycle is non-REM sleep, which is composed of four stages. During the first stage, your eyes move slowly, and your muscles relax with occasional twitches. In the second stage, your heartbeat and breathing slow, and your muscles relax even further. The third stage is deep sleep, where your body is in recovery mode, and your brain activity slows. The fourth stage is REM sleep, where brain activity shoots back up to levels similar to when you are awake. This is when you experience the most intense dreams. During REM sleep, the thalamus is active, sending the cortex images, sounds, and other sensations that fill our dreams.

During REM sleep, the eyes move rapidly behind closed eyelids. However, it is unclear whether the eyes are taking in visual information from the external environment during this stage. While the eyes may still be sensitive to light during REM sleep, it is possible that the brain is focused on processing internal visual stimuli, such as dreams, rather than external visual information.

The sense of sight is not the only sense that remains active during sleep. Hearing is also relatively unaffected, as most people can be awakened by sounds, such as an alarm clock. Additionally, the sense of touch remains active, as people can be awakened by physical stimuli such as being shaken. However, the senses of smell and taste appear to be diminished during sleep, as people rarely report being awakened by odours or flavours.

Your brain actively processes sound

While you sleep, your brain remains remarkably active. It cycles repeatedly through two different types of sleep: rapid-eye movement (REM) sleep and non-REM sleep. During non-REM sleep, your brain waves slow down, and your body transitions into a state of sleep. In the second stage of non-REM sleep, your body and mind slow down even further as you settle into sleep. The third stage of non-REM sleep is deep sleep, during which your body and brain activity slow down significantly.

During REM sleep, brain activity shoots back up to levels similar to when you are awake. Your eyes move rapidly behind closed eyelids, and your breath rate increases. It is in this stage that you dream. While your brain is active during REM sleep, it does not process external sounds in the same way it does when you are awake. Instead, your thalamus, which is responsible for sending and receiving information from the senses to the cerebral cortex, becomes quiet, allowing you to tune out external sounds.

However, your brain can still process sounds during sleep, and this has been demonstrated in sleep studies. The pattern of brain activation associated with auditory stimulation was found to be similar in both wakefulness and sleep, suggesting that sensory processing of sound occurs in both states. This is supported by the fact that certain sounds can awaken someone, regardless of what stage of sleep they are in. For example, young mothers are often awakened by their infants' lightest movements, and people tend to respond to their own names while sleeping. These results suggest that the sleeping brain can detect and process sounds, particularly those that are relevant or familiar.

Overall, while your brain may process sounds during sleep, it is selective about the information it receives. The thalamus acts as a filter, allowing you to tune out external noises and promoting restful sleep.

Your sense of smell is diminished

While it was previously believed that sleep was a passive activity during which the brain was dormant, it is now understood that sleep is a period during which the brain is actively engaged in various activities necessary for life and quality of life. Throughout your time asleep, your brain will cycle repeatedly through two different types of sleep: rapid-eye movement (REM) sleep and non-REM sleep. During non-REM sleep, which is composed of four stages, your body and mind slow down as you settle into sleep. During REM sleep, brain activity shoots back up to levels similar to when you are awake, which explains why REM sleep is associated with intense dreams.

During sleep, your body activity decreases, and your senses are still operating. However, your brain's ability to interpret input slows down, affecting all of your senses, not just vision. In a sleep-deprived state, you may find that you react to auditory stimuli more slowly, and your mental alertness may be impaired. Poor sleep can reduce the acuity of your senses, and your brain's ability to process new information and input is diminished. This is known as brain plasticity, or the brain's ability to adapt to input.

While your sense of smell is still active during sleep, it is diminished compared to when you are awake. Olfactory awareness in humans is low to absent during sleep, and human olfaction does not appear to be sensitive or reliable enough to act as a sentinel system. For example, most people do not recall being awoken by a smell, and most deaths due to fire are due to intoxication rather than being burned, suggesting that smell is diminished during sleep.

However, it is important to note that there is no effective way to turn off our senses during sleep. Instead, our cognitive processing of sensations changes. Our higher brain functions allow us to ignore certain sensory input. For example, the pattern of brain activation associated with auditory stimulation is similar in wakefulness and sleep, suggesting that sensory processing occurs in both conditions.

Your body cycles through REM and non-REM sleep

Sleep is vital for your body and mind. It is a period during which the brain engages in activities necessary for life and closely linked to your quality of life. During sleep, your brain cycles repeatedly through two different types of sleep: REM (rapid-eye movement) sleep and non-REM sleep. The first part of the cycle is non-REM sleep, which is composed of four stages.

During non-REM sleep, your brain is not as active as it is during REM sleep. In the deeper stages of non-REM sleep, your body repairs and regenerates tissues, builds bone and muscle, and strengthens your immune system. Your breathing also slows down, and your blood pressure drops.

After the four stages of non-REM sleep, your body enters REM sleep. During this stage, your eyes move rapidly behind closed eyelids, and brain waves are similar to those during wakefulness. Your breath rate increases, and your muscles become temporarily paralysed as you dream. Dreams typically happen during REM sleep.

Once the REM stage is over, your body starts a new cycle by entering the non-REM stage again. On a typical night, you cycle through four or five times. Each cycle includes three stages of non-REM sleep and one stage of REM sleep and lasts between 90 and 120 minutes.

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