Chronobiotics
Chronobiotics are substances, including many prescription medications and dietary supplements, that influence and alter the body’s circadian (“around the day”) and circannual rhythms (“around the year”). Chronobiotics stand apart from sedatives (which induce drowsiness) and stimulants (which increase alertness). Chronobiotics can reset the circadian clock in the suprachiasmatic nucleus of the brain. The are useful for entraining desynchronized circadian rhythms. They may have a stimulant or sedative function in some cases and for some patients, but that it not the reason doctors prescribe them.
Circadian rhythms are internal biological patterns that are synchronized with the day/night light/dark cycle of the environment. Body temperature, blood pressure and heart rate, hormone secretion, alertness, energy, mood, and weight gain are affected by the body’s circadian rhythm. The circadian rhythm also has an impact upon the effectiveness of certain medications; this is the reason why some medications have instructions as to the specific time they must be taken.
The "clock" part of the brain - the suprachiasmatic nucleus - is rich in cells that have receptors for the hormone melatonin. Two biochemical receptors have been identified: MT(1) and MT(2) Melatonin's influence on the sleep/wake rhythm cycle is thought to be due to its action on those receptors. Other organs in the body have clocks, too, including the retinas and adrenal glands. The suprachiasmatic nucleus is often called the "master clock" (or pacemaker or oscillator) for the body. The body and brain become synchronized with environmental time cues, mainly the presence or absence of light. This adjustment is called entrainment.
Synthetic melatonin derivatives have been created that can activate those receptors and can increase sleep duration in rats.
The most well-known chronobiotic is the hormone melatonin, which was discovered in 1958. All mammals, including humans, secrete melatonin as a response to changing light levels which in turn regulates internal circadian and cannual rhythms. Darkness cues an increased production of melatonin, while light cues a decreased production of the hormone. The amount of melatonin secreted and the sensitivity of melatonin receptors in the body changes during the day, with the highest sensitivity at dusk and dawn.
Melatonin is especially important for the onset of sleep, but it is also necessary for the proper functioning of our cardiovascular, immune, endocrine, and metabolic systems. Cued by darkness, melatonin levels rise which decrease body temperature, heart rate, blood pressure, pupil size, and alertness; all normal body states during sleep. The presence of light reverses this process, resulting in awakening and alertness.
Laboratory studies in vitro and in animals and clinical investigation of people has shown melatonin has two functions. It plays a part in maintaining sleep (and in initiating sleep) and it influences the body's 24-hr circadian rhythms. This hard science has led drug companies to investigate molecules that are similar to melatonin for possible use against insomnia. These drugs are called agonists or analogs.
Today’s hectic lifestyle has resulted in a large number of people with altered circadian rhythm sleep disorders. Sleep disorders affect all aspects of life; including school and work performance as well as a person’s mental and emotional health. Charonobiotic drugs are useful for delayed sleep phase disorder, non-24 hour sleep-wake disorder, and similar sleep disorder. Working or driving with a sleep debt decreases alertness and increases response time, which can lead to injury or even death. A large number of people have resorted to either prescription or over-the-counter sleep aids; however, the use of these sleep aids can contribute to their problem by negatively altering the body’s normal circadian rhythm.
Photoperiodicity
Photoperiodicity refers to the study of how plants and animals respond to daylight. The word was originally used to refer to a plant's timing of flowering with the length of the day, but has now extended to cover a range of activities and behaviors.
The word scotobiology has recently been coined to identify biological study of darkness on living things. It is not clear how scotobiology is distinct from photobiology.
Read about different types of chronobiotic substances.
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