Biorhythms
Questions received:
Responses:
To what, if any, extent is the pineal gland linked to depression and sunlight deprivation?
14th May 1999
The pineal helps to regulate our biological rhythms, linking our physiology and behaviour to external patterns of day and night. If for some reason this process of entrainment is disturbed, we can experience depression, lethargy, and changes in sleeping and eating patterns (Sedgwick, 1998). A familiar example of out-of-synch biorhythms is the feeling of jet-lag after a long-haul flight. You may have noticed that jet-lag is more noticeable if you arrive in a gloomy, wintery environment rather than a bright, sunny one, suggesting that brightness of light is one factor involved in setting biorhythms.
Seasonal and circadian (daily) rhythms are conspicuous features in most biological systems. ‘Clock’ mechanisms regulate many aspects of physiology, metabolism, and behaviour. Our own circadian clocks have an intrinsic period of 23 to 25 hours, and have to be reset frequently (entrained) by environmental cues such as daylight and darkness. The biological clock consists of three main components (Sassone-Corsi, 1998): an input pathway that links the internal cycle to the external light-dark patterns, a pacemaker that generates a daily oscillation, and an output pathway to generate physiological rhythms. Each cell within the pacemaker constitutes a semi-autonomous clock, oscillating with its own characteristics. Considerable progress is being made in understanding the genetic and molecular events which produce these rhythms:
The pineal glands of birds, reptiles, and fish contain photosensitive cells that drive rhythmic hormone production directly according to the light-dark cycle. In mammals, however, the arrangement is more complex and the main pacemaker is located in another part of the brain, the suprachiasmatic nucleus of the hypothalamus. The suprachiasmatic nucleus receives inputs from the retina and also serotonin (5-hydroxy tryptamine) signals from the midbrain. This information about light levels facilitates entrainment of the pacemaker with external light-dark cycles. The hypothalamus then drives the pineal rhythmically by sending noradrenergic impulses via the sympathetic nervous system, taking a rather circuitous route down to the superior cervical ganglion in the neck and then back up into the cranial cavity to the pineal via nerve fibres that course alongside blood vessels. So although the pineal is actually developed from the brain and is attached to it by a stalk, its nerve supply reaches it indirectly.
The pineal converts rhythmical neural patterns into a rhythmical hormonal pattern by producing melatonin from serotonin in inverse proportion to light levels. Thus melatonin levels in the blood are high during the night and low during the daytime. (Recently melatonin has received much media hype as a panacea and an 'antiageing' treatment, but most of these claims are without scientific evidence so far.)
In 1984 Rosenthal and co-workers described a syndrome which they called seasonal affective disorder (SAD), characterised by winter depression (in the northern hemisphere), lethargy and a craving for carbohydrate associated with weight gain. It occurs most commonly in young women during autumn and winter with full remission during the following spring. Recent longitudinal studies show a prevalence of seasonal depressive symptoms in up to 10% of the general population (Gysin, Gysin, and Gross, 1997). SAD becomes rare above the age of 50. Hypotheses on the underlying mechanisms of these behavioural disorders indicate that environmental variables, e.g. climate, latitude, light, and changes in neurotransmitter function that occur naturally with the seasons may be important (Attar-Levy, 1998). During the winter, women increase their nightly production of melatonin, while during the summer nights they produce less. In men the production of melatonin is relatively unchanging through the seasons (Leutwyler, 1995).
Since the 1980s exposure to bright artificial light in the morning has been found effective in the treatment of SAD (Partonen and Lonnqvist, 1998). It is believed that this treatment has an effect on biological rhythms, helping the person to remain in synchrony with the daily light-dark cycle. Not everyone is persuaded by this explanation, however, and there is now an alternative view that the burst of high-intensity light is having an effect on the way serotonin - a neurotransmitter - is working in the brain (Penev, Zee, and Turek, 1997).
References
Attar-Levy, D. (1998) Seasonal depression. [Article in French] Therapie, 53(5), 489-498 (Sep-Oct).
Gysin, F., Gysin, F., and Gross, F. (1997) Winter depression and phototherapy. The state of the art. [Article in Portugese] Acta Med Port, 10(12), 887-893 (Dec).
Hastings, M. (1995) Resetting the circadian cycle. Nature, 376, 296-297.
Leutwyler, K. (1995) Depression’s double standard. Reprinted in Mysteries of the mind, Scientific American (special publication), 53-54.
Partonen, T., and Lonnqvist, J. (1998) Seasonal affective disorder. Lancet, 352(9137), 1369-1374 (Oct 24).
Penev, D.P., Zee, P.C., and Turek, F.W. (1997) Serotonin in the spotlight. Nature, 385, 123.
Saeed, S.A., and Bruce, T.J. (1998) Seasonal affective disorders. American Family Physician, 57(6), 1340-1346, 1351-1352 (Mar 15).
Sassone-Corsi, P. (1998) Molecular clocks: mastering time by gene regulation. Nature, 392, 871-874.
Sedgwick, P.M. (1998) Disorders of the sleep-wake cycle in adults. Postgraduate Medical Journal, 74(869), 134-138 (Mar).
Whitmore, D., and Sassone-Corsi, P. (1999) Cryptic clues to clock function. Nature, 398, 557-558.
20th October 1998
Although the core body temperature is kept remarkably constant by a multitude of homeostatic processes, there is indeed a regular slight cyclical variation during a normal 24-hour period. Our temperature is at its lowest shortly before we wake up in the morning, and highest in the afternoon and early evening. However, the difference is only about 1 Celcius degree. This cyclical change in body temperature appears to be linked to our ‘biological clock’, like so many other body functions. Even if someone remains at rest during the day, this temperature variation is observed. There are also longer term cycles in temperature variation. For example, during the second half of the menstrual cycle a woman’s temperature rises on average by half a Celcius degree compared with the first half of the cycle.
Clearly, if we exercise vigorously or work hard physically, our core temperature will rise, sometimes as much as 3 degrees. (If this happened in a person who is not exercising we would call it fever!) This is because metabolic heat is being generated more quickly than it can be disposed of. The main sources of this heat are the skeletal muscles and the liver. Also, if we are exposed to extreme changes in environmental temperature, either much higher or lower than usual, then this may be reflected by a change in core temperature as the homeostatic processes adapt.
Although the temperature for the abdominal, thoracic, and cranial structures remains tightly controlled and relatively stable, it is worth remembering that the skin and subcutaneous tissue experience much wider fluctuations in temperature, although usually without damage.
The daily changes in body temperature and other parameters are referred to as diurnal variation (from the Latin diurnalis: recurring every day, having a daily cycle) or circadian variation (from the Latin circa about and dies day: occurring in about 24-hour cycles), and there is intensive research into the origin of these cycles and how they are controlled.
Circadian cycles are seen in most living things, from micro-organisms to plants to animals. The ‘clocks’ driving these cycles are thought to be webs of gene expression within cells (Sassone-Corsi, 1998). So far, three genes with significant links to the circadian cycle have been identified. Although these clocks have intrinsic rhythms that are close to 24 hours in length, they need frequent resetting to keep the living organism in harmony with its environment, and the most effective stimulus for resetting a biological clock is light (Hastings, 1995; Boivin et al, 1996). If you have experienced long-haul flights you will be familiar with the jet-lag that occurs as your biological clock resets itself to the new time-frame.
There are three parts to a circadian clock: an input pathway for resetting the clock, a pacemaker that generates an oscillation with a period of approximately 24 hours, and an output pathway that converts the oscillations into changes in physiology and behaviour (Schibler, 1998).
In
adult vertebrates there are three significant clocks: the retina, the pineal
gland, and the suprachiasmatic nucleus of the hypothalamus. In our case, it
seems that the suprachiasmatic nucleus is the main clock driving cyclical
changes such as temperature variations (Takahashi, 1996). This cluster of about
10,000 small neurons releases a variety of neuropeptides which, amongst other
activities, regulate the release of melatonin by the pineal gland.
It is interesting that biological clocks also have a role during development of the embryos, influencing the rate of cell division and pathways of cellular differentiation. Indeed, it has been suggested that the processes which control cell division (the cell cycle) provide an example of the first circadian clock to arise during evolution.
Click here for notes about measuring body
temperature - NOT ADDED YET.
References
Boivin, D.B., Duffy, J.F., Kronauer, R.E., and Czeisler, C.A. (1996) Dose-response relationships for resetting of human circadian clock by light. Nature, 379, 540-542.
Hastings, M. (1995) Resetting the circadian cycle. Nature, 376, 296-297.
Sassone-Corsi, P. (1998) Molecular clocks: mastering time by gene regulation. Nature, 392, 871-874.
Schibler, U. (1998) New cogwheels in the clockworks. Nature, 393, 620-621.
Takahashi, J.S. (1996) Ion channels get the message. Nature, 382, 117-118.
How do biorhythms affect industrial safety?
22nd February 2000
Biorhythms can be an issue for certain groups of industrial workers, particularly those working at irregular times and during the night. Safety can be compromised if a person is carrying out tasks that are potentially dangerous at a time when their biorhythms are not correctly synchronised with the pattern of work.
Many of the physiological processes within us follow circadian rhythms. This rhythmicity derives from an internal biological clock as well as our rhythmic environment and rhythmic habits (see previous Nurse Minerva answers on the subject of biorhythms.) Generally, there is a synchrony between these different processes and influences, but this can be disrupted by irregular hours of work and by long-distance travel, producing undesirable side-effects (Minors and Waterhouse, 1987).
When people work at night, their biorhythms adapt only partially (Weibel, Follenius, and Brandenberger, 1999). The degree of shift in melatonin secretion differs markedly from one person to another, and there are changes in the patterns of secretion of cortisol, thyroid stimulating hormone, growth hormone, and prolactin. People with weak biorhythms are more adaptable to shiftwork when young but are more likely to develop intolerance to shiftwork later in life compared with people with high-amplitude biorhythms (Smolensky and Reinberg, 1990).
There is evidence that shiftwork, and in particular night work, can have negative effects on work performance, health and social well-being, leading in the short-term to sleep disorders and accidents and in the longer term to increased risks of gastrointestinal, cardiovascular, reproductive, and psychological problems (Scott and LaDou, 1990; Tepas and Carvalhais, 1990; Luna, 1997; Costa, 1999). This is therefore an important management issue. Night workers tend to have less than normal sleep on their work days and are generally unable to compensate sufficiently for this on their non-workdays (Tepas and Carvalhais, 1990).
References
Costa, G. (1999) Shift work and health. [Article in Italian] Med Lav, 90(6), 739-751 (Nov-Dec).
Luna, T.D. (1997) Air traffic controller shiftwork: what are the implications for aviation safety? A review. Aviation and Space Environmental Medicine, 68(1), 69-79 (Jan).
Minors, D.S., and Waterhouse, J.M. (1987) Circadian rhythms and their application to occupational health and medicine. Rev Environmental Health, 7(1-2), 1-64 (Jan-Jun).
Scott, A.J., and LaDou, J. (1990) Shiftwork: effects on sleep and health with recommendations for medical surveillance and screening. Occupational Medicine, 5(2), 273-299 (Apr-Jun).
Smolensky, M.H., and Reinberg, A. (1990) Clinical chronobiology: relevance and applications to the practice of occupational medicine. Occupational Medicine, 5(2), 239-272 (Apr-Jun).
Tepas, D.I., and Carvalhais, A.B. (1990) Sleep patterns of shiftworkers. Occupational Medicine, 5(2), 199-208 (Apr-Jun).
Weibel, L., Follenius, M., and Brandenberger, G. (1999) Biologic rhythms: their changes in night-shift workers. [Article in French] Presse Med, 28(5), 252-258 (Feb 6).