Here in my part of the United States we experienced the switch to Daylight Savings Time last night. That is to say, we transitioned from “Standard Time” to “Daylight Time”. I live in California, so that means going from Pacific Standard Time (PST) to Pacific Daylight Time (PDT). I hate the daylight savings time switch. I have always hated daylight savings time whether we were moving our clocks forward in the spring, or back in the fall. I used to say that it was like getting jetlag without the benefit of going anywhere. This morning, as I lay in bed, I realized that that little joke obscured the true reason why I find this time change so unpleasant.
I was thinking about what it would be like if we hadn’t gone to daylight time last night, but instead, I had taken a red-eye flight to somewhere one time zone to the east. For me that could mean flying from San Francisco to Denver. When I landed in Denver I would have reset my watch to one hour later – ugh. But the transition to DST is worse than that. Why? Is it just because I am now in Denver and able to visit friends and see and do things that I cant do back home? That is what I used to think, but that is not the real reason.
I realized that at any given moment there are three kinds of time going on that are relevant to my normal life. There is what I call “clock time”, the time displayed on a clock, there is “solar time”, the place where the sun is in the sky (which is the time that would be displayed on a sundial), and there is “circadian time”, the time that it subjectively feels like to my body.
If I flew from San Francisco to Denver I would experience jetlag because my circadian time would be out of sync with clock time. But it wouldn’t be that bad because the local clock time and solar time would be in sync. For me the sun would rise and set 23 hours after the last time I had seen it rise and set. My body would adapt relatively quickly, taking its cue from the sun, causing my circadian time to come into alignment with both the local solar time and clock time.
The transition from PST to PDT is much worse than that. This is because the morning after the switch both my circadian time and solar time would be out of sync with clock time. The clock would tell me that the sun rose 23 hours after the prior sunrise. But that wouldn’t be true. I would experience (approximately) 24 hours between sunrise on the morning before the switchover and the morning after the time change.
At this time of year in California, the sun rises 1 or 2 minutes earlier each day (because of elliptical orbits and the tilt of the earth’s axis, the change in time of sunrise from one day to the next is not constant.) That means that it will be April 24th before the solar time and the new DST clock time are back in sync again. My body is able to adjust to the daily 1 or 2 minute change in solar time that occurs throughout the year without my even noticing it. My body is able to adjust to travel between time zones because the solar time in the new time zone corresponds to the local clock time, helping me correct my circadian clock. But I have a terrible time with the daylight savings time change because only the clock time has changed – my circadian time and the solar time are both out of sync with what the government tells me the time is now.
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