Introduction
A biological rhythm is any cyclic change in the level of body chemicals or function.
Regulation
- Endogenous: Controlled internally.
- Biological clocks: e.g., the Suprachiasmatic Nucleus (SCN) – located in the hypothalamus and receives direct input from the eyes. The hypothalamus continually measures the light exposure of the eye and adjusts the sleep/wake cycle time.
- Melatonin: The pineal hormone that induces sleep. Its production is mainly at night and is longer during winter nights.
- Exogenous: Controlled externally.
- e.g., light exposure to the eyes controls the sleep/wake cycle.
Period: Refers to the measured time from the start of a cycle to the start of a new cycle.
TYPES OF CYCLES
- Annual cycle: Occurs yearly.
- Daily cycle: Night & day.
- Lunar cycle: Monthly cycles.
- Tidal cycle: Related to the tide.
Properties Of Biological Rhythms
- They have self-sustaining pacemaker mechanisms.
- Maintain their normal cyclicity even in the absence of environmental cues.
- Are unaffected by metabolic poisons or inhibitors.
- Are genetically transmitted.
- The biological clocks are reset by environmental cues called Zeitgebers.
- The ability of a biological clock to reset itself is called entrainment.
Types Of Biological Rhythms
- Circadian rhythm: Takes 24 hours to complete a cycle e.g. sleep/wake cycle, body temperature, pattern of hormone secretion, etc.
- Ultradian rhythm: Occurs in less than 24 hours. E.g., eating.
- Infradian rhythm: Takes longer than 24 hours to complete a cycle e.g., the female menstrual cycle.
Pattern Of Activity In A Biological Rhythm
- Diurnal: Organisms active during daytime.
- Nocturnal: Organisms active in the night.
- Crepuscular: Organisms active at dusk, dawn, or twilight.
- Arrhythmic: No regular pattern.
Application Of Biological Rhythm
- Jet lag: Temporary sleep disorder due to travel across multiple time zones.
- Morningness and eveningness: [link to a blog post about chronotypes]
- Pheromones: Cause synchronization of the menstrual cycle in females living together.
Clinical Correlates
- Cardiac arrhythmia: Any deviation from the normal rhythm (sinus rhythm) of the heart. Are of two types:
- Tachycardia: Faster than normal heart rate.
- Bradycardia: Slower than normal heart rate.
- Treatment:
- Melatonin capsule: Can be used to treat sleeplessness.
- Phototherapy: Use of bright light to alter the sleep/wake cycle.
