Two kinds of rhythm
Biorhythm vs circadian rhythm
They sound alike and are often confused, but they are different things. A circadian rhythm is a real, measured cycle of about 24 hours that daylight resets every morning. A biorhythm is a cycle of weeks counted from the day you were born, and when it was tested it was not found.
- Biorhythm
- 23 · 28 · 33 days
- Circadian rhythm
- about 24 hours
The body clock
What a circadian rhythm is
The word comes from the Latin circa diem, "about a day", and was coined by the physiologist Franz Halberg in 1959. It names the roughly 24-hour cycle that runs through sleep and waking, body temperature, hormone levels and much else. In people and other mammals a small cluster of cells in the brain, the suprachiasmatic nucleus, keeps the master clock, and light reaching the eyes each morning sets it to the day.
Kept away from daylight, in a cave or a laboratory, the clock keeps running on its own, at a little over 24 hours in most people. In 2017 Jeffrey Hall, Michael Rosbash and Michael Young shared the Nobel Prize in Physiology or Medicine for finding the genes and proteins that make it run.
The theory
What a biorhythm is
Biorhythm theory holds that three cycles, of 23, 28 and 33 days, begin on the day you are born and run unchanged for life. Nothing resets them, and nothing in the body has been found that keeps them. They were proposed between the 1890s and the 1920s and tested from the 1970s, and the studies did not find them. Are biorhythms real? sets out the research.
Side by side
| Biorhythm | Circadian rhythm | |
|---|---|---|
| Length | 23, 28 and 33 days, the same for everyone | About 24 hours, a little different from person to person |
| Set by | The date of birth, once | Daylight, every morning |
| Changes | Never | Shifts when the light changes: jet lag is the clock catching up |
| Kept by | Nothing identified | Clock genes, and a master clock in the brain |
| Evidence | Tested from the 1970s; a 1998 review of 134 studies found no effect | Measured for decades; a Nobel Prize in 2017 |
Chronobiology
Other rhythms that are real
Chronobiology, the science of biological timing, has found rhythms shorter and longer than a day. Each was found by measuring it, and each is kept by something that responds to the world outside: light, the tide, the moon, the length of the day.
A night
Through a night of sleep, the brain passes through its stages in cycles of roughly 90 minutes.
The tide
Crabs and other shore animals keep a tidal rhythm of about 12.4 hours, in time with the sea, even when they are moved away from it.
The moon
Many corals spawn together on a few nights a year, timed by the moon.
The year
Migrating birds keep a yearly rhythm, set by the changing length of the day.
Why the two get confused
The names are close, and both are about cycles in the body.
Some writers also use "biorhythm" loosely for any rhythm of the body, so an article about sleep may call the circadian rhythm a biorhythm. On this site, and in BioCalendar, the word means only the theory's three cycles of 23, 28 and 33 days.
BioCalendar
Where BioCalendar stands
BioCalendar counts the three biorhythm cycles from your date of birth and says plainly that they are a prompt for reflection, not a measurement. It does not track sleep, read your body clock, or tell you when to sleep, eat or work. For questions about sleep, a doctor is the right person to ask.
Sources
- Halberg, F. (1959). Physiologic 24-hour periodicity; general and procedural considerations with reference to the adrenal cycle. Zeitschrift für Vitamin-, Hormon- und Fermentforschung, 10, 225–296.
- Czeisler, C. A., et al. (1999). Stability, precision, and near-24-hour period of the human circadian pacemaker. Science, 284, 2177–2181.
- The Nobel Assembly at Karolinska Institutet (2017). The Nobel Prize in Physiology or Medicine 2017: Jeffrey C. Hall, Michael Rosbash and Michael W. Young.
- Hines, T. M. (1998). Comprehensive review of biorhythm theory. Psychological Reports, 83, 19–64.
Continue the story
Keep going
- 01What is a biorhythm?A biorhythm is a cycle said to begin at birth and repeat on a fixed period: 23 days physical, 28 emotional, 33 intellectual. The theory and the evidence.
- 02Are biorhythms real?What the studies found when they tested biorhythm theory against accidents, sport, exams and deaths, why people use it anyway, and how we frame it.
- 03Biorhythm vs astrologyBiorhythms and astrology both read a day from your birth, and neither held up when tested. What they share, where they differ, and how we use one of them.