Tags: Chronotype, Circadian rhythm, Core body temperature, Cortisol, DLMO, DSPS, Eveningness, Jet lag, Melatonin, MEQ, Morningness, Shift work, Sleep architecture, Sleep deprivation
“Well, I don’t like getting up before seven either,
but I have to, and I do.”
Implication: and you can, too.
But, is that true?
The article on Wikipedia about Chronotypes, morningness/eveningness, reports what researchers know so far about these normal variations, which have been studied since about 1970 and are measured by the Horne-Östberg questionnaire, the MEQ, a short version of which can be found online at Bruce Logie’s interesting site.
It can be interesting to compare normal evening types with what we know about people with Delayed Sleep-Phase Syndrome.
- like to sleep in and don’t like to go to bed early.
- are more alert in the evening than just after awakening, as opposed to morning types.
- can take a nap at 10 a.m. or noon after a night with less sleep than usual, while morning types generally don’t want a nap until 2 p.m. or later.
- experience both Dim-Light Melatonin Onset (DLMO) and the minimum of the daily cortisol rhythm later (clock time) than morning types.
So far, it sounds like people with DSPS are evening types, as the properties above apply to both groups. However, normal evening types:
- after starting a new routine requiring them, for example, to start work earlier than before, will adjust their sleep-wake schedules to the new times within a few days.
- awaken spontaneously earlier in their circadian phase than morning people; that is, the interval between the low point of the body temperature and wake time is shortest in evening types. In people with DSPS, it’s notably much longer than average.
People with DSPS do not adjust to a new schedule easily, if at all.
- Evening types have a core body temperature which is a bit lower than average, both day and night. Is this also true for people with DSPS?
- Evening types have a melatonin profile which declines much more slowly after midpoint, as compared with morning types. Is this also true for people with DSPS?
- Evening types take a long time to “get going” after awakening. In relation to the timing of spontaneous awakening, the following points contribute to this for evening types, and possibly also for people with DSPS:
- the timing of lowest body temperature,
- the timing of the cortisol minimum,
- the timing of melatonin offset, and
- the slower decline of blood levels of melatonin.
We have a disorder which, without treatment, forces us to fall asleep even later than evening types. Simply trying to enforce conventional sleep and wake times does not advance the circadian markers. It seems almost impossible to wake us much earlier than our pre-programmed wake time (as my siblings will attest). The disorder is chronic, changing little or not at all after the age of 20.
When normal chronotypes shift their schedules, all the body’s rhythms catch up and are synchronized to each other within a few days. In DSPS, the dissynchrony may continue as long as the shifted, “unnatural” schedule lasts, even for years or decades, leading to physical and psychological disorders.
It’s clear that people with DSPS who (try to) work days, have much the same set of problems that many shift workers have, whether these always work nights or are on a rotating schedule. However, shift workers’ problems receive sympathy and understanding while people with DSPS are commonly stereotyped as undisciplined and lazy. Dagan again (PDF, page 7), on adolescents, points out that “[f]requently, the patients’ parents, teachers, doctors, or psychologists believe that the patients’ biological sleep-wake problem and the accompanying dysfunction at school are motivational or psychological in nature, a belief that during the years, the patients tend to adapt themselves. This attitude toward CRSD patients, to which [they have] been subjected since early childhood or adolescence, adds psychological distress to the practical difficulties of coping with life.”
Like normal people, we do adjust (entrain) to the earth’s 24-hour rotation, but, without treatment, we don’t “learn” to wake up at a conventional, early time of day.
Tags: Australian study, Circadian rhythm, Core body temperature, Cortisol, Jet lag, Melatonin, Serotonin
- Core Body Temperature (CBT) and, incidentally, the temperature of the soles of our feet
- Melatonin, “the hormone of darkness”
During our day, CBT bobs up and down. It’s up into the fever range when we exercize or take a hot bath. It drops quite low if we lie down for a minute or two. Charting CBT throughout one’s waking hours while living normally is difficult to interpret and thus not very meaningful.
Those who volunteer for studies where they must recline almost motionless and stay awake in near-darkness in order to have their temperatures measured continually, are usually paid for their trouble. There are many such charts to be found. The illustration above is from this lecture slide by Dr. Bjørn Bjorvatn in Norway. It is easy to follow, with degrees celsius on the vertical axis and the time from 6 p.m. to 6 p.m. on the horizontal.
If one is sleeping at the time one’s body prefers to sleep, CBT declines very steadily and evenly for hours as shown in the illustration, from a while before bedtime until about two hours before awakening. Then, very abruptly, it starts climbing on a somewhat steeper slope than the decline.
Each person’s body “knows” this curve and when it should occur in relation to the 24 hour day. Each person’s body has its own rule about when awakening should occur: precisely X minutes after the temperature minimum. If your temperature minimum occurs at 5 a.m. and you awaken spontaneously at 7 a.m., you are very normal and about average.
Researchers don’t agree on what makes this long temperature decline start each day. It is clearly related to the secretion and blood level of the hormone melatonin, but which triggers which, or does something else trigger both?
In any case, all that body heat has to go somewhere. If you’ve noticed that your feet get hot just as you’re getting sleepy, you’re very observant. The heat which will be excess as you “hibernate” for the night is escaping. That’s OK, just let it.
According to an Australian study from 2001 we are sleepy when our feet are at their warmest, an average of 4 degrees C. above normal. This occurs shortly after onset of melatonin in the blood, when the core body temperature has just started its decline.
Just for fun, see a figure from that study (and a link to it) in my next post.
In the post after that: more about melatonin and the effect of light.