Saturday, December 17, 2011

I'm sleeping better, but why?

I love my Zeo Sleep Manager, a cool device that measures my brain activity in order to help understand the quality of my sleep: the amount of refreshing REM versus Deep sleep, versus plain-old-ordinary light sleep, and of course the all-up total sleep each night. I started using it in mid-2010, and have used it to track my sleep most nights since then.

I don't have any particular sleep issues, though I was intrigued by the idea that I may be used to a particular quality of sleep, and maybe I just think I'm doing okay. Well, after using Zeo for this long I can't say that I feel all that different -- I feel fine, and always have -- but feelings are hard to measure objectively. Is there a way to measure my sleep quality more scientifically?

Zeo uses a measure they call "ZQ", which tries to combine a bunch of aspects of sleep into a single number you can compare across nights. After analyzing my data for the past eighteen months, I've noticed that my ZQ number seems to be going up over time, for no apparent reason.  At least, I don't think I've been changing anything about myself: I just go to bed when I'm tired, and wake up when it's morning. I haven't deliberately tried to change anything about myself.

But even without trying, my ZQ seems to have improved over the past year.  Since the amount of daylight varies throughout the year, I analyzed my numbers date by date.  (I don't have data for every single night, so my analysis skips the nights when I don't have a datapoint for both years).

Here's a chart that shows the difference in my ZQ compared between given dates in 2010 and 2011.

Sprague ZQ 2011 vs 2010

As you can see, there seems to be a clear trend of my ZQ improving, and for the past few months it's improved significantly – on the order of 20 or more points per night. I wonder why?