Understanding the Basics of Sleep
Sleep is a mysterious time during which the brain rests and repairs itself, muscles relax, and memories are consolidated. It also plays a role in regulating the body’s hormones and immune system. Insufficient or irregular sleep over time can cause both short- and long-term health problems, including heart disease, high blood pressure, weight gain, diabetes, depression, stroke, and even cancer.
Although the ubiquity of sleep in nature suggests it’s essential to life, the precise biological mechanisms that underlie its functions are still being unraveled. Humans have a special challenge in this respect because our behavioral and physiological criteria for classifying sleep are often at variance with one another. Nevertheless, a combination of behavioral and physiological evidence – namely, that we’re transiently inactive, unresponsive to external stimuli, and accompanied by a specific type of EEG activity known as sleep spindles – has traditionally been used to characterize human sleep.
The first phase of sleep is typically defined as a low-voltage mixed-frequency EEG tracing with a considerable representation of theta waves (4-7 Hz). The appearance of this pattern of brain activity, along with momentary interruptions of the natural sleeping cycle and a relatively stable core body temperature, has been taken to be characteristic of sleep (Carskadon and Dement, 2005). In the thalamic gating hypothesis, sensory inputs are blocked by thalamic relays at the level of the neocortex, which leaves the sleeper unaffected. This ‘unconnectedness’ is thought to contribute to the sense of unresponsiveness experienced by people who are sleeping.
At a higher level, the EEG data suggest that sleep is characterized by a general decrease in cortical activation, with the exception of the areas of the brain responsible for processing visual and auditory information (Krueger et al., 2019). Then, at the brainstem level, a series of interconnected neurons in the area called the locus ceruleus become active, with an increase in calcium ions and a reduction in neurotransmitters such as dopamine and serotonin. This is believed to represent the onset of stage 1.
The second phase of sleep involves more pronounced increases in brain activity, including the production of new memory pathways. It is believed to be a time of’restful learning’ and is associated with improvements in both attention and decision making. It’s also important for emotional regulation and coping with stress. Research shows that people who get enough sleep do better at work, are healthier, and enjoy a greater quality of life. This is because they have a stronger immune system, which can fight off infections and diseases like the common cold. Sleep also allows the proteins and cells in our immune systems to remember foreign invaders so that they can better recognize and destroy them when we encounter them again, such as in the form of a new germ or virus. These benefits of a good night’s sleep have been called ‘the gift of a full life’.