Tuesday, August 16, 2005

Bipolar? Avoid night shift.

I have only touched a little bit on the topic of interaction between the circadian clock and Bipolar Disorder before. You can find stuff online like this, this and this.

What I'd like to focus on right now is an interesting hypothesis, called the Social Rhythm Stability Hypothesis (SRSH). One of the originators of this hypothesis is Cindy Ehlers, now at Scripps (See her website here. See one of her papers, e.g,. Ehlers CL, Kupfer DJ, Frank E, Monk TH. Biological rhythms and depression: The role of Zeitgebers and zeitstorers. Depression 1:285-293, 1994.).

According to SRSH, the core problem in bipolar disorder is instability of regular daily patterns of activity. As I have mentioned before, light-dark cycle is the most powerful environmental cue (Zeitgeber) that entrains circadian rhythms, but is not the only one. Cold-blooded animals entrain to temperature cycles, and several other cues have been demonstrated in various species. Humans are extremely sensitive to social cues. Getting a brief social cue about time of day resets the human clock even if all other cues (e.g., light) are removed.

Social rhythms in bipolar patients remain stable if the social Zeitgebers are stable. People around you have their own regular schedules, both at work and at home. Your pets ask to be walked on the leash at exactly the same time each day. This is good.

What is bad is that bipolar patients are extremely sensitive to disruptions of social schedules. Ehlers and others coined a new term - Zeitstorer - to describe a person or a social demand that throws off the regularity of the daily pattern of activity. Whenever you start a new relationship, get a new job, buy a new pet, or have a baby, your schedule is disrupted. If you are bipolar, this will result in wild cycling until you get used to the new routine.

Now consider getting a job that demands you work on a rotating shift. You are getting a Zeitstorer every week! Bad idea if you are a bipolar sufferer. Even a steady night shift is a disturbance as such a pattern in hard on one's body and one also tends to shift back to daytime activities over the weekends.

Is SRSH a realistic hypothesis? I say - why not? It has been shown long ago that circadian rhythm disturbances are both causes and symptoms of bipolar disorder. During depressive episodes, the phase is advanced - you usually become more of a "lark", you wake up earlier and have a lesser total amount of activity per day. During manic episodes, one is more of an "owl", staying up late and increasing total daily activity.

Lithium is one of the rare chemicals that has been shown to affect the period of the circadian clock. For instance you can see here that even individual neurons of the mammalian clock (the SCN) lengthen their period when exposed to lithium. If you have paid attention in class you know that period determines phase. Increasing the period delays the phase (if your period is 25 hours you will wake up 1 hour after dawn every day, if your period is 23 hours you will wake up 1 hour before dawn every day - that works in humans only if you exit civilization and live out in the wild). Thus lithium phase-delays the rhythm which is already phase-advanced, thus, hopefully, putting back into a normal phase.


Blogger Hsien Lei said...

Just leaving a trackback to this post from the Genetics and Public Health Blog - Grand Rounds #47: The Best of This Week's Medblogging.

Coturnix at Circadiana is hosting this week's Grand Rounds #47 - From one room to another. Traveling from "the classroom" to the "hospital room" and ending in the "smoke-filled backroom" (I'll hold my breath while I'm in there) can be pretty exhausting but well worth the trek....

5:35 AM  

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