Blog Posts 1–5, based on the United States of America Department of the Army Field Manual: Holistic Healing and Fitness, describe what really matters for your child’s sleep. If sleep is an important enough topic for national defense than surely sleep should be considered a serious topic for parenting!
Non-human primates mainly evolved within the tropical zone between the Tropic of Cancer and the Tropic of Capricorn where there is little variation in the duration of daylight (the photoperiod). The closer the latitude is to the equator, the less variation of the photoperiod. At the equator, sunrise and sunset varies little from 6am and 6pm respectively. There is a strong biologic and ancient development of sleep-wake rhythms driven by the rotation of the earth on its axis creating alternating light (daytime) and dark (nighttime) cycles.
Non-human primate mothers always had close contact with their babies for breast feeding and safety. Also, adult non-human primates, in the Tropical Zone, evolved a strong sleep-wake rhythm approximating two 12-hour cycles of darkness and daylight. As primates moved farther from the equator, reproductive fitness favored those individuals who were able to adapt to seasonal changes in the photoperiod and temperature. While early human primates are thought to have originated from of the Tropical Zone in Africa, much of later human evolution occurred further North from the equator in Eurasia.
Moving North, with increasing latitude from the equator there are two main variables to consider:
In other words, the farther from the equator, the more variation there is of the photoperiod and the more pronounced seasonality is with alternating seasons of increasing and decreasing photoperiod length and temperature.
Evolution selected for those individuals migrating North who were able to adapt to living far from the equator with the challenges of seasonality regarding changes in the duration of the photoperiod and temperature. For example, the human control of fire permitted light in the darkness and warmth in the cold and subsequently, this allowed some adult individuals to adapt to later bedtimes.
Young children are larks, not owls (Blog Post 75). I think that very young humans initially have strong biologic sleep-wake rhythms that favor early bedtimes (Blog Posts 7 and 70) which reflects our biologic heritage from the Tropical Zone. With biologic maturation of the human brain, driven by evolution, bedtimes become later. This genetically driven circadian development of sleep-wake rhythms in the child matures over the first few years as naps disappear and then the bedtime becomes later. However, babies and very young children still need early bedtimes.
(To be continued)
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