Biological Rhythms

Please note that this section contains my personal notes from my readings on this topic.

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To better understand the importance of maintaining sleep schedules, let’s look at how four distinctive biological rhythms develop.  From Dr. Marc Weissbluth, M.D. of Healthy Sleep Habits, Happy Child (1987), pages 50 – 51:

1. Immediately after birth, babies are wakeful, then fall asleep, awaken, and fall asleep a second time over a ten-hour period.  These periods of wakefulness are predictable and not due to hunger, although what causes them is unknown.  Thus a partial sleep / wake pattern or rhythm emerges immediately after birth.
2. Body temperature rhythms appear and influence sleep / wake cycles.  Body temperature typically rises during the day and drops to lower levels at night.  At six weeks of age, temperature at bedtime is significantly higher than later in the night.  After six weeks of age, as temperatures fall more with sleep, the sleep periods get longer.  By twelve to sixteen weeks, all babies show consistent temperature rhythms.  It is exactly at six weeks of age when evening fussiness or crying begins to decrease from peak levels and night sleep becomes organized, and it is at twelve to sixteen weeks when day sleep patterns become established.
3. A third pattern is added by three to six months of age, when the hormone cortisol also shows a similar characteristic rhythm, with peak concentrations in the early morning and lowest levels around midnight.  Interestingly, a part of the cortisol secretion rhythm is related to the sleep / wake rhythm and another part is coupled to the body temperature rhythm.
4. Melatonin rhythmicity is a fourth pattern to consider.  Initially, a newborn has high levels of circulating melatonin, which secreted by the mother’s pineal gland and crosses the placenta.  Within about one week, the melatonin that came from the mother has disappeared.  At about six weeks of age, melatonin begins to reappear as the baby’s pineal gland matures.  But the levels are extremely low until twelve to sixteen weeks of age.  Then melatonin begins to surge at night, and the hormone appears to be associated with evolving sleep / wake rhythms by about six months of age.

Even at only a few months of age, then, interrelated, internal rhythms are already well developed: sleep / wake pattern, body temperature, and cortisol and melatonin levels.  In adults, it appears that a long night’s sleep is most dependent on going to sleep at or just after the peak of the temperature cycle.  Bedtimes occurring near the lower portion of the temperature cycle.  Bedtimes occurring near the lower portion of the temperature cycle result in shorter sleep durations.

Shift work or jet travel in adults, or parental mismanagement in children, might cause disorganized sleep.  What is “disorganized sleep”?  When you are awake but your body clock is in the sleep mode or when you crash from exhaustion when your body is in the awake mode, then your wakefulness or sleep is occurring out of phase with many biological rhythms.  The result is poor-quality sleep or poor-quality wakefulness.

Related Content:

• Sleep’s Impact on the Brain
• Sleep’s Influence on “Intelligence” and School Performance
• Benefits of Good Sleep Habits
• Five Elements of Healthy Sleep
• Prevent Poor Sleep Habits
• Biological Rhythms