Rhythm and Blues: Biological Clock Depends on Light

This powerful clock is actually very small. It consists of about 15 000 neurons (that’s small!) in an area of the brain called the suprachiasmatic nucleus (SCN) which is the size of a grain of rice. The SCN sits in the hypothalamus directly above the optic chiasm (where the right and left optic nerves meet). Knowing its position, you will not be surprised to read that it takes its cues from light. In this way the SCN orchestrates all sorts of daily rhythms: body temperature, blood pressure, heart rate, hormone levels and sleep-wake times.

Biologists have been looking for many years for what makes the body clock itself tick. How does the SCN direct the nature of so many physiological and metabolic functions? Many of these are cyclic changes that occur every 24 hours. They are controlled by an interplay of numerous molecular factors which ensure the accuracy of the body clock. They are organised in complex feedback loops which involve gene transcription and the events that follow it. Several of the genes involved in this daily rhythm have been identified and called such inventive names as clock, but then others have less obvious names such as Bmal1 and Rev-erbα. In fruitflies, zebra and some mammals contain this body clock in their peripheral tissues. This indicates the presence of a ‘synchronisation web’ that coordinates timing in all of the tissues, and it depends on the daily rhythm of light and dark. In humans, we have ‘evolved’ to the point that the body clock has become centralised into the SCN in the brain, rather than being found throughout the body.

The passing of the daylight hours is detected by the retina in the eye, and transmitted to the SCN. During the daytime, the SCN fires powerfully and through a series of chemical steps suppresses the release of melatonin (the sleep hormone) from the pineal gland. At night, light can no longer activate the SCN, its firing drops, the pineal gland can secrete melatonin and it flows into the blood stream making us sleepy.

In the last 5 years scientists have started finding connections between disturbed rhythms and disorders such as Alzheimer’s and schizophrenia. Certain hallmark symptoms of these disorders may stem directly from faulty internal time keeping. Interrupted sleep or abnormal sleep times are a common symptom of many psychological disorders. In depression 40-65% of depressed people suffer severe sleep disorders. Age can also upset the balance between sleep and emotional well-being. Older people often say that they have a harder time sleeping through the night than they did when they were younger. Dutch researchers, led by Eus van Someren, have been studying nerve cells in the SCN which produce vasopressin, a hormone that controls salt and water balances in the body. They found that vasopressin in the brain also regulates some of our daily rhythms such as temperature, wakefulness and activity levels. As the body grows older, apparently the number of cells that produce vasopressin decrease. As a result, older people find it increasingly difficult to keep their internal clock in step with the day-night cycle.

This problem is even more dramatic in Alzheimer’s, which appears to slow circadian rhythms – the body clock always runs late. Alzheimer’s patients have a postponed activity pattern. Because of this body clock delay they tend to roam around the house late at night when their caregivers want to sleep…. Apparently the majority of Alzheimer’s patients do not get put into hospital because of failing memories but because they ‘haunt’ the house at night. Sadly, once they get put into institutions their health rapidly declines.

Van Someren and his team decided to  test whether Alzheimer’s patients could shift their sleeping patterns in response to light therapy. In rats, the number of SCN neurons that produce vasopressin decrease with age, also leading to sleep disorders. But if elderly rats are exposed to bright light during the day, vasopressin production increases and their sleeping pattern starts to become more normal. The cells aren’t destroyed they just become dormant (they go to sleep!!). Care facilities for the elderly are often very gloomy with low light levels. The researchers installed powerful lamps in 12 Dutch old peoples’ homes, 6 of which received full-spectrum artificial light (emitting the frequencies needed to adjust the body clock), and 6 with normal artificial light. Some patients also took melatonin in the evening in addition to receiving light by day. Often older people have decreased melatonin production, and with the onset of dementia, melatonin synthesis decreases even more. This thus further weakens an important input to the SCN.

As you might expect,  the combination of light therapy and melatonin produced the best results.  The longer subjects received treatment, the better they slept. Their moods improved, particularly in those prone to depression. But another finding also surprised the researchers: the full spectrum lamps helped slow down mental deterioration too! It worked as well as the ‘cholinesterase inhibitors’ that many old people were taking for declining memory.

So, as your recent summer holiday starts fading into memory, promise yourself that you’ll get out into the fresh air every day to top up on your natural light needs. Your memory, mood, health and family will thank you for it!

References:
David G. Harper et al. ‘Differential circadian rhythm disturbances in men with Alzheimer disease and frontotemporal degeneration.’ Archives of General Psychiatry, 2001, volume 58, no. 4, pages 353-360.

Russell Foster & Leon Kreitzman. ‘Rhythmms of Life’. Profile Books 2005.

K. Wulff et al. ‘The suitability of actigraphy, diary data, and urinary melatonin profiles for quantitative assessment of sleep disturbances in schizophrenia: aa case report.’ Chronobiology International, 2006, volume 23, nos. 1-2, pages 485-495.

Eus van Someren. ‘Improving actigraphic sleep estimates in insomnia and dementia: how many nights?’ Journal  Sleep Research, 2007, volume 6, no. 3, pages 269-275.

M. Hoekert et al. ‘Comparison between informant-observed and actigraphic assessments of sleep-wake rhythm disturbances in demented residents of homes for the elderly.’ American Journal Geriatric Psychiatry, 2006 volume 14, no. 2, pages 104-11.