April 20, 2013

Analyzing Baby Sleep

A little over 2 months ago I decided to start tracking Nicki’s sleep. At the time she wasn’t sleeping very well and I wanted to have a dataset I could analyze.

Histogram of the number of hours Nicki spends sleeping.
It may appear like a left skew distribution, but that’s because I was using a sub optimal bedtime during the initial few weeks of my study. Without those weeks her histogram shows a normal distribution with mean 11:30-12:00.

For this analysis I mostly looked at correlation. Correlation shows the statistical relationship between two sets of numbers. It ranges from -1 to 1. Negative correlation [-1,0) shows two variables are inversely related. As one increases, the other decreases. Positive correlation shows two variables tend to increase or decrease together. The closer to 0, the weaker the correlation.

Correlation(Time put down, Time spent asleep) = -.72
When I put her to bed earlier, she tends to sleep longer.

The time I put Nicki down for bed is correlated with how long she sleeps – earlier bed times mean more sleeping! That makes intuitive sense. My circadian rhythm wakes me up at certain points, provided I’ve slept a decent amount. I’m now in the habbit of waking up at 7:00 am, regardless of what time Nicki wakes up. (Mommy misses sleeping in until noon on the weekend.) Nicki could be the same way. Earlier bedtimes mean there’s more hours between when she goes down and when she typically gets up, which could correspond to longer sleep intervals.

Every baby book I own says “Early to Bed Late To Rise“. In other words, put the baby to sleep early and she will sleep in longer. What do my numbers show?

Correlation(Time put down, Time Woken Up) = -.21
When I put her to bed earlier, she tends to wake up later.

So yes, she does tend to sleep in longer on days she goes down earlier, but it’s weak correlation. It could be that the relationship is weak, or that there are other factors at play. One possible factor is day light savings time. Specifically the position of the sun. We’re in the middle of Spring, sunrise is getting earlier and Nicki tends to wake up around sunrise. If I take a weekly average of her wake up time, I see it inching forward for the first three weeks.

Another aspect of sleep I care about is how long it takes her to fall asleep. The books all say over tired babies have a harder time falling asleep. Was it true for Nicki?

Correlation(Time put down, Number of Minutes needed to fall asleep) = 0.4
When I put her to bed earlier, she takes less time to fall asleep.

My analysis shows that, at least for Nicki, earlier bed times lead to better sleeping.

Still asleep after sunrise. Love the bear on her butt!

Of course, Correlation does not imply Causation. There could be other factors at play. Our bed time is between 7/7:30. On days she’s extra tired, she goes down a little earlier. Less sleepy, and bed time is closer to 7:30. A tired baby is more likely to fall asleep quickly and sleep longer.

If I were to do a true study I’d have to randomize her bedtime. That means some nights putting a wide awake baby down, and some nights trying to keep a tired baby awake. I may love data, but even I’m not that crazy. Still, it’s neat to see Nicki’s sleep numbers.

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