24 Hour Daylight
24 Hour Daylight
Posted by Mary Morrell

Tuesday, 21 Apr 2009 13:07
Hi, I am back to talk about sleep again.

It is now 52 days since the team set off on the ice, and the hours of daylight they are experiencing have changed considerably.

Day one: here in London the sun rose at 07:00 and set at 17:41, so we had 10 hrs 41 mins of daylight, whilst the team were only experiencing a few hours of sunlight per day. Day 39: 8th April we experienced 13 hrs 26 mins of daylight, as the sun rose at 06:19 and it set at 19:45. When Pen, Ann and Martin started the expedition they were in almost perpetual darkness. By the 9th April at their latitude of 83o 50’ N, Longitude 128 o 50’ W they were experiencing almost 24 hours of daylight. In my last entry (11th March) I mentioned that ambient light has an important influence on sleep because it is the main factor that regulates the body’s internal clock, the so called circadian rhythms.

What are circadian rhythms?
The term circadian means ‘about a day’ and it is used to describe the activity of the cells in our body that are responsible for functions such as the regulation of temperature, digestion and the secretion of hormones e.g. cortisol which is a stress hormone, melatonin which among other things promotes sleep, and growth hormone which is released at night.

Image 1 - Variation in body temperature over a 24 hour period; the falling body temperature between 21:00 and midnight promotes sleep, whereas the rising temperature from 05:00 onwards promotes wakefulness. Note the small dip in temperature at 13:00 which shows this is an ideal time for napping! Reproduced on Wikipedia from the 1911 Encyclopædia Britannica Eleventh Edition
Where is the body clock?
The principal body clock is located in the suprachiasmatic nucleus (SCN) in the hypothalamus. The main factor which regulates the clock is ambient light. Information passes from the retina at the back of the eye to the SCN. These external cues are called ‘Zeitgebers’ (from the German ‘time givers’).

Image 2 - The position of the suprachiasmatic nucleus in the brain (web.bvu.edu)

In humans there is a natural variation in the cycle length of the body clock. Some people have a circadian rhythm that is less than 24 hours (e.g. approx 23 hours) and some people having a longer circadian rhythm (e.g. approx 25 hours). Research shows that most of us tend to have a longer cycle of 24 hours 42 mins. This means that every 24 hours we have to ‘reset’ in order to live ‘in phase’ with the 24 hour day.

In environments such as the Arctic, where light levels do not change over a 24 hour period the body clock will ‘free run’, and if left unchecked our natural sleep time would become later each day. Of course this will not happen to the ice team as they have watches to tell them when to go to bed.

Image 3 - Sleep patterns are plotted for 45 days, over two 24 hour periods. The solid dark line represents the sleep time and the dashed line is wakefulness. Note that during the free running period the sleep time gradually moves forward each day (represented by the triangle). www.sleephomepages.org/sleepsyllabus/fr-g.html

Do we all have circadian rhythms?
Recently the amount of research into circadian rhythms has increased because more people are travelling across time zones, which exposes them to Jet lag, and because shift work has become more common with our 24/7 lifestyle. Jet lag is a normal experience caused by the delay in resetting the circadian rhythms following a rapid change in time cues. The seasonal changes in time cues are slower, so our bodies have time to adapt. Interestingly scientists have suggested that animals living in the Arctic, where these is a lack of time cues, may have evolved without circadian rhythms.


Image 4 - Six reindeer in Svaldbard (78 degrees North) were found to have no circadian rhythms (Nature. 2005;438:1095-6; http://scienceblogs.com/clock/2007/07/circadian_rhythms_or_not_in_ar_1.php). This may be an example of evolution which the ice team unfortunately don’t have.

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