Biological rhythms are cyclical patterns of physiological and behavioral activities that repeat over time intervals ranging from hours to years. Examples include circadian (daily), circalunar (monthly), and circannual (yearly) rhythms that coordinate processes like sleep/wake cycles, hormone secretion, and hibernation with environmental cycles like day/night, lunar phases, and seasons. These rhythms are controlled by an internal biological clock and can persist even without external cues, running on a natural but not always 24-hour cycle. Chronobiology is the study of biological timekeeping and rhythms.

1. BIOLOGICAL RHYTHMS: IT’S A MATTER OF TIME
Text: Rhythms of Life
Russell Foster and Leon Kreitzman

2. • What are Biological Rhythms?
• What are Rhythms?

3. Pattern
Sequence
Regularity
Progression
Time
Measure
Beat
What is a rhythm?

4. Rhythm = sequence of events that repeat themselves in
the same order and with the same time interval, over and
over again.
Biological rhythm: a biological event or function with a
pattern of activity that is repeated over and over again at
a constant time interval.

6. Biological Rhythms
Heart rate
Breathing
Hormone secretion
Menstrual cycle
Body temperature
Sleep/wake cycle
Time
What are examples of Biological Rhythms?

7. • Biological Rhythms are the product of an internal biological
timekeeping system which is controlled by a biological clock
•Chronobiology
•

8. Two Broad Categories of Biological Rhythms
•high frequency –
•Ultradian
•Those that have a constant relationship with environmental
rhythms –
•Have a geophysical counterpart

9. ENVIRONMENTAL RHYTHMS
Semi-Daily Rhythms
•Tidal
Daily Rhythms
•Solar
Monthly
•Lunar
Quarterly
•Seasons
Annual
Longer than a year

10. Spinning of earth on its axis 24h – solar day
Movement of earth around sun 365 days – year
Tilting of earth on its axis seasons
Movement of moon around earth 24.53 days – lunar month
24.8h – lunar day
Geophysical

11. Types of biological rhythms with a geophysical counterpart:
Period
Length
Name
Chronobiological
Name
Example
12.4 h tidal CIRCA TIDAL crab activity on shoreline
29 days monthly CIRCA LUNAR menstrual cycle, marine reproduction.
365
days
yearly CIRCANNUAL Hibernation, many reproductive cycles.
24 h daily
CIRCADIAN
(circa + diem)
sleep-wake cycle and many others
infradian

12. What Kind of Organisms Have Biological Rhythms?

13. What is the purpose of having a biological timekeeping system?
Promotes organism’s ability to survive by coordinating its activities
with changes in the environment
Coordinates internal processes

14. STROMATOLITE

15. Many organisms have several kinds of biological rhythms

16. Alexander the Great
4th Century BC
Tamarind Tree

17. de Mairan, 1729
endogenous

19. Du Monceau 1759
Circadian rhythms do not depend on temperature changes
Linneaeus 1751
Circadian rhythms are genetically determined

20. Linneaeus 1751

21. de Candolle 1832
When not exposed to environmental day/night rhythms plant
leaves opened and closed on a 22 – 23 h cycle instead of a 24 h
cycle
Free-running rhythm

22. •Circadian rhythms are endogenous.
•Endogenous rhythms are not exactly 24h.
•The periods of Circadian rhythms are genetically
determined.
•Endogenous rhythms are temperature-compensated

23. Bees – 1910 Forel
1929 Beling
1950s Renner
Drosophila – 1950s Pittendrigh

24. PACEMAKER
TARGET TISSUE
BIOLOGICAL RHYTHMS
(overt rhythms)

25. ZEITGEBER
TRANSDUCER
PACEMAKER
INTERMEDIATE ELEMENTS
TARGET TISSUES
BIOLOGICAL RHYTHMS
Endogenous, free-running
rhythm

26. Activity (locomotor) rhythm

27. Chronobiology
Biological rhythm
Ultradian
Infradian
Circadian
Circatidal
Circalunar
Circannual
Endogenous
Free-running
Pacemaker
Target tissue
Overt rhythm
Nocturnal
Diurnal
LL
DD
LD 12:12
Study of biological timekeeping
Cyclical, repeated variation in a biological function
High frequency – repeats many times in a day
Repeats at intervals much longer than 24 hours
Approximately a day (24 hours)
Approximately every 12.4 hours ( with the tide)
Approximately once a month
Approximately once a year
Internally generated rhythm
Not synchronized to external signals
A structure that generates a rhythm
Tissue whose function is regulated by the pacemaker
A visible, measurable rhythm
Active at night
Active during the day
Constant light
Constant darkness
12 hours of light; 12 hours of dark