(day). The human CR is slightly more than 24 hours in length and is controlled by the SCN. Adjustment to the CR, termed entrainment, is facilitated by external cues called zeitgebers (time givers), such as light, temperature, and social interaction. There are two other rhythms besides CR:
1 Infradian rhythms are greater than 24 hours, such as menstruation and seasonal depression.
2 Ultradian rhythms are shorter than CR such as heart rate, nostril dilation, and appetite.
The Gut Microbiome
The role of the gut microbiome (GM) on sleep as well as other physiologic activities has been a developing area of interest from the standpoint of the impact it may have in humans. It has been shown that alterations in the GM may impact sleep, blood pressure, our circadian rhythm, and even cognition [43, 44]. In the future, our sleep and the management of sleep disorders may be impacted by the interaction between the gut and brain [45], often referred to as the gut‐brain axis.
Figure 2.5 Release of melatonin.
Conclusion
Human sleep is a complex interrelationship between a wide variety of neurotransmitters and how they impact the brain and one another. Having a basic understanding of neuroanatomy as well as neurobiology is also important as it helps to understand brain function in sleep and wakefulness. Our sleep is no longer viewed as an alternate state of consciousness but rather a separate state of being with unique properties that are designed to facilitate improved health, mood, memory, and well‐being. Over time a better understanding of the neurotransmitters involved in sleep will no doubt occur, resulting in a better understanding of sleep. Ultimately, sleep is controlled by the brain and is for the brain.
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