There are 24 hours in a day because ancient Egyptians split daylight into 12 parts with shadow clocks and the night into 12 parts by rising stars. Greek astronomers later made the 24 hours equal, the Babylonian base-60 system gave each hour 60 minutes, and equal hours became part of everyday life after mechanical clocks appeared in 14th-century Europe.

Key takeaways

  • Egyptian astronomers marked the night with 12 star risings and, by about 1500 BC, shadow clocks split daylight into 12 parts (in NIST’s account, 10 parts plus two twilight hours): together, the first 24-part day (Scientific American, NIST).
  • Those early hours were seasonal: a summer daylight hour ran longer than a winter one, because each daylight was always cut into 12 (Scientific American).
  • Hipparchus proposed 24 equal, equinoctial hours in the 2nd century BC, but ordinary people kept seasonal hours until after mechanical clocks appeared in 14th-century Europe (Scientific American).
  • The 60 minutes and 60 seconds come from Babylonian base-60 mathematics, written into astronomy by Ptolemy around AD 150 (Scientific American, NIST).
  • Since 1967 the second has been defined by the caesium atom, not by the turning Earth, and the human body clock runs on a period that averages 24.18 hours, so daily light keeps it in step (BIPM; Czeisler et al., Science, 1999).

Who decided there are 24 hours in a day?

No one person did: the 24-hour day grew out of Egyptian timekeeping, which counted 12 hours of daylight and 12 hours of night. NIST metrologist Michael Lombardi, writing in Scientific American, credits Egypt as the first civilization to divide the day into parts like hours.

The night came first. Egyptian astronomers watched a set of 36 stars, now known as decans, that divided the circle of the heavens into equal parts. According to Lombardi, 18 of them could mark the passage of a night, but three were given to each twilight, when stars were hard to see, which left 12 for full darkness. In the New Kingdom (1550 to 1070 BC) the scheme was simplified to 24 stars, 12 of them marking the night.

Star tables painted inside coffin lids, mostly from about 2100 BC, preserve the scheme: columns headed by decans, each with 12 rows that historians Neugebauer and Parker read in the 1960s as the hours of the night. Scholars Sarah Symons and Elizabeth Tasker explain in Scientific American that the name decan comes from the Greek for ten, because each one headed a 10-day week of the Egyptian civil year. Symons cautions that hours told this way would have been shorter than 60 minutes and probably quite irregular; her view is that the tables are more like almanacs than practical clocks. Whether the decans worked as hours at all is still debated.

Daylight followed. NIST describes a portable Egyptian shadow clock from about 1500 BC that divided a sunlit day into 10 parts plus two twilight hours. The accounts differ in detail: Lombardi describes a T-shaped sundial of the same era that divided sunrise to sunset into 12 parts. Either way, 12 daylight parts plus 12 night hours make 24.

Why 12 and not 10?

Historians are not sure why the Egyptians chose 12. Lombardi gives the two leading ideas: the roughly 12 lunar cycles in a year, or counting the finger joints of one hand with the thumb.

Were ancient hours all the same length?

No. For most of history an hour was one twelfth of the daylight or one twelfth of the night, so its length changed with the seasons. These are called seasonal, or temporal, hours.

Era How the day was divided Equal hours?
Egypt, by about 1500 BC 12 night hours by stars; daylight in 12 parts by shadow clock (or 10 + 2 twilight hours) No, seasonal
Sumer and Babylon, from about 2000 BC Base-60 numbers for astronomy and angles Not for daily life
Hipparchus, 2nd century BC 24 equinoctial hours Yes, for astronomers
Ptolemy, about AD 150 Degrees split into 60 minutes, minutes into 60 seconds For astronomy
Europe, 14th century Mechanical clocks strike equal hours Yes, in everyday life
France, 1793 to 1795 Decimal day: 10 hours of 100 minutes Yes, briefly
1967 Second defined by the caesium atom Yes, atomic

Summer hours were long and winter hours short. Around the 2nd century BC the Greek astronomer Hipparchus proposed dividing the whole day into 24 equinoctial hours, the equal hours that daylight and night share at an equinox. Scholars used them; ordinary people kept seasonal hours for centuries. Lombardi notes that hours of fixed length became commonplace only after mechanical clocks appeared in 14th-century Europe. A gear train ticks at the same rate in June and December, so the clock, not the sun, began to set the hour. For what happened when railways tied those clock hours across a whole country, see the Day of Two Noons.

Why are there 60 minutes in an hour?

Because of Babylonian mathematics. The Sumerians developed a base-60 number system around 2000 BC and the Babylonians inherited it, according to Scientific American. Sixty divides evenly by 2, 3, 4, 5, 6, 10, 12, 15, 20 and 30, which made fractions easy.

Astronomers used base 60 for angles first. In the Almagest, around AD 150, Ptolemy divided each degree into 60 minutes and each minute into 60 seconds; NIST notes these units later found their way into medieval mechanical clocks. Minutes came late to clock faces: mechanical clocks that displayed minutes appeared near the end of the 16th century.

Has anyone tried a different number of hours?

Yes. France tried a decimal day during its Revolution. The Fondation Napoléon notes that the republican calendar decreed on 24 November 1793 came with a day of 10 hours of 100 minutes, and that on 7 April 1795 the Convention suspended the decimal-hour decree indefinitely, after less than a year in use. Decimal clocks are rare for that reason. Reforms of the calendar itself have fared better, as when ten days vanished in 1582.

Traditional Chinese timekeeping divided the day into 12 shichen, units of two hours each. The Horniman Museum in London holds two Chinese incense spirals marked at regular intervals in that system. Beyond Earth, the count changes again: a day on Mars, called a sol, runs 24 hours 39 minutes 35 seconds, according to NASA’s Mars24 notes.

How long is a second, and why do leap seconds exist?

Since 1967, the second has been defined by atoms, not by Earth. The 13th General Conference on Weights and Measures (CGPM) defined it as 9,192,631,770 periods of the radiation from a transition in the caesium-133 atom, replacing the earlier astronomical definition.

Earth did not agree to keep pace. NIST explains that Earth’s rotation is irregular and, over long intervals, gradually slowing, so leap seconds have been added since 1972 to keep Coordinated Universal Time (UTC) within 0.9 seconds of the time set by Earth’s turning. In 2022 the 27th CGPM decided that the allowed gap will be raised in or before 2035, aiming to keep UTC continuous for at least a century; Scientific American reported Russia voted against the change.

Does the body run on a 24-hour day?

Close, but not exactly. In a 1999 Science study, Charles Czeisler and colleagues measured the rhythms of melatonin, core body temperature and cortisol in healthy young and older adults living in carefully controlled light, and found the intrinsic period of the human circadian pacemaker averages 24.18 hours, about 24 hours and 11 minutes, in both age groups.

That small gap is why daylight matters. The National Institute of General Medical Sciences (NIGMS) notes that light and dark have the biggest influence on circadian rhythms: the master clock in the brain, the suprachiasmatic nucleus, takes its cue from light reaching the eyes and keeps clocks in organs across the body in sync. An American Academy of Sleep Medicine review notes that light around dawn resets the clock earlier and light around dusk resets it later, so morning light nudges a slightly long body clock back toward 24 hours each day. Without light cues, rhythms free-run on their own, non-24-hour cycle.

The lineage is worth seeing whole. The sun gave the first hours; societies agreed on shared clock hours, from Egyptian star tables to caesium; science then found the clock inside us, recognized with the 2017 Nobel Prize to Jeffrey Hall, Michael Rosbash and Michael Young, and Owaves, founded in Encinitas in 2013, brought those rhythms together on one 24-hour circle, My BodyClock.

Your day on My BodyClock

Why Are There 24 Hours in a Day?. My BodyClock for Night of Wednesday, October 28 in Encinitas: Flow at 6:30 AM (Wake, open the curtains); Move at 6:45 AM (Walk in morning light); Eat at 7:30 AM (Breakfast); Flow at 8 AM (Get ready, commute); Work at 8:30 AM (Focused work); Move at 12 PM (Midday walk outside); Eat at 12:30 PM (Lunch); Work at 1:15 PM; Flow at 4:30 PM (Head home, errands); Love at 5 PM (Family time); Eat at 6 PM (Dinner); Play at 6:45 PM (Board game). Center Photo: A weathered stone sundial on a pedestal in a green garden in clear natural light.
Your day on My BodyClock. Build it in Owaves.

The day above on My BodyClock is a light-anchored autumn weekday, built for a body clock that runs a little long. It opens at 6:30 AM with a walk in morning light, the strongest signal for pulling the clock back to 24 hours. A midday walk catches bright light again, meals land at regular hours, and the evening dims: reading by lamplight, screens off at 9 PM, lights out at 9:30 PM. The same wake time each day, as in how to fix your sleep schedule, matters most when the clocks change at the end of daylight saving time.

Build your own version in Owaves: the World’s First Wellness Planner, Powered by Your Body Clock. Take the Body Clock Type quiz to find your natural timing, then plan the day on My BodyClock in Owaves for iPhone. On Android? Join the waitlist.

FAQ

Why are there 24 hours in a day and not 10?

Ancient Egyptians counted 12 hours of night by rising stars and 12 hours of daylight with shadow clocks, a base-12 habit that may come from lunar cycles or finger joints. France tried a 10-hour decimal day from 1793 but suspended it in 1795.

Who invented the 60-minute hour?

No single inventor. The base-60 system came from the Sumerians and Babylonians around 2000 BC, and Ptolemy used it to split degrees into 60 minutes and 60 seconds around AD 150. Clocks showing minutes appeared near the end of the 16th century.

When did hours become equal in length?

Hipparchus proposed 24 equal hours in the 2nd century BC, but they stayed an astronomer’s tool. Equal hours became part of everyday life after mechanical clocks appeared in 14th-century Europe.

Is the human body clock exactly 24 hours?

No. A 1999 Science study by Czeisler and colleagues found the intrinsic human circadian period averages 24.18 hours, in both young and older adults. Daily light, especially in the morning, keeps it in step with the 24-hour day.

Sources

  1. Michael A. Lombardi (NIST), answering for Scientific American. Why is a minute divided into 60 seconds, an hour into 60 minutes, yet there are only 24 hours in a day? Scientific American, 5 March 2007. https://www.scientificamerican.com/article/experts-time-division-days-hours-minutes/
  2. National Institute of Standards and Technology. A Walk Through Time: Early Clocks. https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time/walk-through-time-early-clocks
  3. Sarah Symons and Elizabeth Tasker. Decoding the Star Charts of Ancient Egypt. Scientific American, 1 October 2015. https://www.scientificamerican.com/article/decoding-the-star-charts-of-ancient-egypt/
  4. National Institute of Standards and Technology. Second (SI redefinition). https://www.nist.gov/si-redefinition/second-introduction
  5. BIPM. Resolution 1 of the 13th CGPM (1967): SI unit of time (second). https://www.bipm.org/en/committees/cg/cgpm/13-1967/resolution-1
  6. National Institute of Standards and Technology. Leap Seconds FAQs. https://www.nist.gov/pml/time-and-frequency-division/leap-seconds-faqs
  7. BIPM. Resolution 4 of the 27th CGPM (2022): On the use and future development of UTC. https://www.bipm.org/en/-/resolution-cgpm-27-4
  8. Elizabeth Gibney (Nature). The Leap Second’s Time Is Up: World Votes to Stop Pausing Clocks. Scientific American, 22 November 2022. https://www.scientificamerican.com/article/the-leap-seconds-time-is-up-world-votes-to-stop-pausing-clocks/
  9. Fondation Napoléon. Pendule squelette d’époque révolutionnaire. https://www.napoleon.org/histoire-des-2-empires/objets/pendule-squelette-depoque-revolutionnaire/
  10. Horniman Museum and Gardens. Incense spirals, object 15.69. https://www.horniman.ac.uk/object/15.69/
  11. Czeisler CA, Duffy JF, Shanahan TL, et al. Stability, Precision, and Near-24-Hour Period of the Human Circadian Pacemaker. Science 284(5423):2177–2181, 25 June 1999. https://doi.org/10.1126/science.284.5423.2177
  12. National Institute of General Medical Sciences. Circadian Rhythms (fact sheet). https://www.nigms.nih.gov/education/fact-sheets/Pages/circadian-rhythms
  13. Sack RL, Auckley D, Auger RR, et al. Circadian Rhythm Sleep Disorders: Part I, Basic Principles, Shift Work and Jet Lag Disorders. An American Academy of Sleep Medicine Review. Sleep 30(11):1460–1483, 2007. https://doi.org/10.1093/sleep/30.11.1460
  14. NASA Goddard Institute for Space Studies. Mars24 Sunclock: Technical Notes on Mars Solar Time. https://www.giss.nasa.gov/tools/mars24/help/notes.html
  15. Nobel Prize Outreach. The Nobel Prize in Physiology or Medicine 2017: press release. https://www.nobelprize.org/prizes/medicine/2017/press-release/

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