Sleep: Learn It 2—What is Sleep?

What is Sleep?

sleep

Sleep is distinguished by a state of low levels of physical activity and reduced sensory awareness. Sleep is characterized by certain patterns of activity in the brain that can be visualized using electroencephalography (EEG). Different phases of sleep can be differentiated using EEG as well.

 

Sleep is regulated by circadian rhythms and homeostatic mechanisms (Siegel, 2008). Homeostatic regulation of sleep is evidenced by sleep rebound following sleep deprivation:

  • Sleep deprivation is the state of not getting enough sleep, either in duration or quality, resulting in negative effects on physical and mental functioning.
  • Sleep rebound refers to the fact that a sleep-deprived individual will fall asleep more quickly during subsequent opportunities for sleep.

Regulating Sleep

Sleep-wake cycles seem to be controlled by multiple brain areas acting in conjunction with each other. Some of these areas include the thalamus, the hypothalamus, and the pons:

  • The hypothalamus contains the SCN—the main biological clock of the body—in addition to other nuclei that, in conjunction with the thalamus, regulate slow-wave sleep.
  • The pons is important for regulating rapid eye movement (REM) sleep (National Institutes of Health, n.d.).

Sleep is not only regulated by these structures in the brain—it is also regulated by hormones.

Sleep regulation by hormones

Sleep is also associated with the secretion and regulation of a number of hormones from several endocrine glands including:

  • melatonin: Melatonin is a hormone that is naturally produced by the pineal gland in response to darkness (Figure 1). Melatonin plays a key role in promoting healthy sleep patterns and helps regulate the body’s circadian rhythm. When the brain senses darkness, it signals the production of melatonin, which helps induce drowsiness and promote sleep. Recent research suggests that melatonin may have other important roles in the body beyond sleep regulation, but these effects are not fully understood. For example, some studies suggest that melatonin may have antioxidant properties and may even help regulate immune function.[1][2]
  • follicle stimulating hormone (FSH) and luteinizing hormone (LH): During sleep, the pituitary gland secretes both FSH and LH, which are important in regulating the reproductive system (Christensen et al., 2012; Sofikitis et al., 2008).
  • growth hormone: During sleep, the pituitary gland also secretes growth hormone, which plays a role in physical growth and maturation as well as other metabolic processes (Bartke, Sun, & Longo, 2013).
An illustration of a brain shows the locations of the hypothalamus, thalamus, pons, suprachiasmatic nucleus, pituitary gland, and pineal gland.
Figure 1. The pineal and pituitary glands secrete a number of hormones during sleep.
Have you (or someone you know) ever experienced significant periods of sleep deprivation because of simple insomnia, high levels of stress, or as a side effect from a medication? What were the consequences of missing out on sleep?

  1. Hardeland et al., 2006
  2. Carrillo-Vico, A., Lardone, P. J., Alvarez-Sánchez, N., Rodríguez-Rodríguez, A., & Guerrero, J. M. (2013). Melatonin: buffering the immune system. International journal of molecular sciences, 14(4), 8638–8683. https://doi.org/10.3390/ijms14048638