The "Clocks" of the Body

Hello everyone!

I would like to start the second week of my internship by talking about what I am researching about in detail. 

The body is complex and many things, such as temperature, blood pressure, oxygen levels, etc. must be regulated. To accomplish these things, our bodies have internal “clocks” that control biological rhythms. One of the rhythms is the circadian rhythm, which controls the sleeping-waking cycle every 24 hours. It also controls the ups and downs of biological patterns, like body temperature and blood pressure changes during sleep and wakefulness. 

For example, the body temperature falls right before we sleep and rises when we are about to wake up. Biologically, there is a cluster of cells in the hypothalamus of the brain, the suprachiasmatic nucleus, which controls the circadian clock. 

There are four (maybe five) distinct stages of the sleep cycle. Stage 1 is the stage of sleep where we are between wakefulness and sleep. The most predominant waves in the brain during this stage are theta waves. The body temperatures starts dropping and muscles start relaxing. The eyes also move from side to side. People in Stage 1 sleep are not aware of their surroundings, but can easily be woken up.  Additionally, not everyone experiences State 1 sleep in the same way.

Stage 2 is the first stage of “true sleep.” It lasts about 10-25 minutes. Heart rate and breathing is slower than when we are awake. Muscle activity decreases and consciousness fades away. The brain’s activity is irregular in that large, slow theta waves are present along with sleep spindles. These are shorts bursts of brain activity. Most time is spent in this stage than any other.

During Stage 3, or deep sleep, we are unresponsive to the outside environment. Stage 3 is longer during the first part of the night and decreases as we keep alternating between the different stages throughout the night. During this stage, our brain waves are predominantly large, slow delta waves. This stage used to be split into Stage 3 and Stage 4 sleep. Stage 4 was initially defined as when delta waves were most common. We are generally difficult to awaken and during this stage, many people start sleepwalking.

During REM (Rapid Eye Movement) Sleep, the motor cortex of the brain is active. Our eyes move rapidly from side to side and we dream. Also, during this stage, muscles are relaxed. Body temperature, blood pressure, heart rate, and breathing speed all increase. The sympathetic nervous system is very active during this stage. For this reason, REM sleep is sometimes referred to as paradoxical sleep.

The sleep cycle repeats itself approximately every 90 minutes. Further into the night, REM sleep gets longer, while Stage 3/4 sleep gets shorter and eventually ceases altogether, as shown by the image below:

Sleep patterns are affected by various things, including genetics and the environment. Any stage of the sleep cycle can be affected. In upcoming posts, I will talk about some common sleep disorders.

Until next time!

~Shubhangi