What is a biological rhythm?
A biological rhythm is an activity that repeats in a regular cycle in a living thing. These rhythms are adaptations: they help the organism do the right thing at the right time, such as feeding when food is available or avoiding the hot midday.
Rhythms come from an internal clock inside the organism (in humans, a small part of the brain). The clock keeps running even in constant darkness, but it drifts a little. Light resets it every day, so it stays in step with the real day.
Types of adaptive rhythms
- Daily (circadian) rhythm, about 24 hours: sleeping and waking, body temperature, flowers opening by day, owls hunting at night. "Circadian" means "about a day".
- Tidal rhythm, about 12.4 hours: shore animals like crabs and clams hide or feed with the rise and fall of the sea.
- Lunar (monthly) rhythm, about a month: some corals and worms release eggs on a certain moon phase; the human menstrual cycle is also about a month.
- Annual (seasonal) rhythm, about a year: leaf fall, hibernation, migration, breeding season.
Rhythms can be endogenous (driven by the inside clock) and are tuned by exogenous signals such as light and temperature.
Photoperiodism
Photoperiod is the length of daylight in a 24-hour day. Photoperiodism is the response of living things to it. Day length is a very safe signal: it changes in the same way every year, while temperature and rain can be tricky.
In plants, a pigment called phytochrome senses the light and the length of the dark period. What really counts is the length of the unbroken night.
Photoperiodism in plants
- Long-day plants (for example spinach, lettuce, wheat) flower when days are longer than a critical day length.
- Short-day plants (for example chrysanthemum, rice, soybean, cotton) flower when days are shorter than the critical length.
- Day-neutral plants (for example tomato, cucumber, maize) flower whatever the day length.
Growers use this: flower farmers cover chrysanthemums with black cloth to give long nights, or switch on lights to give long days, so flowers appear for a festival.
Photoperiodism in animals
Animals also measure day length. As days lengthen in spring, many birds begin singing, build nests and lay eggs; as days shorten in autumn, many birds migrate, mammals grow thick fur and some animals prepare to hibernate. Hormones from the brain change with the amount of light and turn these actions on or off.
Key formulas and definitions
- Circadian rhythm โ 24 hours
- Tidal rhythm โ 12.4 hours
- Photoperiod = length of daylight in a day
- Long-day plant flowers: day > critical length
- Short-day plant flowers: day < critical length (really, night > critical length)
Worked examples
1. A plant flowers only when the day is shorter than 12 hours. What kind of plant is it?
A short-day plant.
2. In June a place has 14 h of daylight. How long is the night? Will a long-day plant (critical 13 h) flower?
Night = 24 โ 14 = 10 h. The day (14 h) is longer than 13 h, so the long-day plant flowers.
3. A crab hides in the sand twice a day with the high tide. About how many hours between two high tides?
About 12.4 hours, a tidal rhythm.
4. A flower farmer wants a short-day plant to bloom in summer when days are 15 h long. What can he do?
Cover the plants with black cloth for part of the day so that the day is under the critical length (the night is long enough).
Common mistakes
- Thinking plants count days. They measure the length of the night (the dark).
- Thinking a rhythm needs the Sun at all times. The internal clock keeps going without light, but light resets it.
- Mixing up long-day and short-day: the name tells the day length that gives flowers, not how long the plant lives.
- Thinking temperature alone times migration. Day length is the main reliable signal.