Development of the human sleep patterns

Development of the human sleep patternsWhat it really means when we are talking about babies “sleeping through the night” (from September 22, 2005)

Trixe Update is a blog that is very unusual. First, just looking technically, the posts go from top to bottom instead of the latest post being on the very top. Second, the whole blog is devoted to the day-by-day growth and development of Trixie, from birth until about the age of two.
The graph on the right (and there are many different graphs there) shows the sleep-wake cycle. Unfortunately, the first four months – the most interesting months – are missing (I hope they have another kid and do this again from Day 1). But even without the missing data, you can see how the sleep rhythm gradually becomes more and more coherent. At the age of two, Trixie has a clear bimodal pattern of sleep: a long bout throughout the night and a shorter bout around mid-day.
Notice the 16-months time point. Until that time, Trixie was sleeping TWO naps per day in addition to a nicely consolidated night sleep.
Now, look at the first ten months or so. Trixie is sleeping much more during the night than during the day, but there are so many daytime naps and so many awakenings at night, that the actograph appears very messy.
If the data for the first four months were available, you could see the complete absence of a daily rhythm of sleep. The whole 24-hour period looks like the daytime period at 4-10 months of age.
Take a closer look at that 4-10 months period. Look at the onsets and offsets of sleep episodes. It appears as if there are several clocks, each with a different period (some shorter some longer than 24 hours), each trying to drive a separate rhythm of sleep and wakefulness. This is even more apparent in newborns during the first six weeks of life.
The notion here is that various multiple circadian oscillators within the human suprachiasmatic nucleus (SCN) are still, at this age, not coupled to each other, and that it may take as long as several months postnatally for the pacemaker to produce a single unified circadian output.
Exposure of the infant to a light-dark cycle (full daylight during the day and pitch darkness during the night) may speed up the development of the consolidated pattern.
Mother’s exposure to a light-dark cycle will also entrain her circadian clock. This, in turn, will entrain her rhythm of nocturnal secretion of melatonin. It has recently been found that mother’s milk contains melatonin. It is well established in other mammals (not yet in humans, though) that daily exposure to melatonin entrains and consolidates circadian rhythms in the fetal and neonatal offspring.
Thus, both mother’s and infant’s exposure to a robust light-dark cycle are likely to aid in quicker consolidation of the baby’s sleep-wake cycle – something that is a major goal of all parents!

5 responses to “Development of the human sleep patterns

  1. my guess would be that if something like a several independently acting internal clocks exists during some stage of development, than we should be able to see and characterize patients in which the tuning or coordination of these clocks was not completed succesfully. i am not aware of any older children or adults resembling anything like a babies sleep patterns. or does the narcolepsia have some rhytmicity?

  2. Sleep fragments with old age as well. Also, jet-lag is essentially desynchronization of multiple internal clocks.

  3. ok, but these are still not the examples of the sleep disorder as a result of the failure of tuning as a developmental milestone. maybe this comparison is not the best, but i could compare it with the unsuccesful closure of the neural tube that results in spina bifida.

  4. The gradual consolidation of multiple rhythms into a single rhythm is not pathological – it is the normal way the circadian system in humans develop. Fragmentation that comes with old age also appears to be a normal aspect of aging and not neccessarily a pathology, though we can do things to change the pattern in order to raise the quality of life in the elderly. Likewise, exposure of infants to light-dark cycles and an otherwise well-scheduled life speeds up the process of concolidation.

  5. maybe i am not properly understood. i dont deny your claims, but my point was that if any noxious agens interferes with that process of gradual consolidation (which i presume must be highly coordinated), than we might expect to see some phenotypes that could eventually unmask specific role of each of these “sleep modules”, if i could use that term.