Lesson 6: Biological Clock and Timing in Animals

Vocabulary

• Exogenous Rhythm - biological cycle controlled by external factors

• Endogenous Rhythm - it’s an internally driven biological cycle regulated by the biological clock, operating without the need for an environmental stimulus.

• Biorhythms - It’s the natural cycles that control changes in our body's functions and behaviors

• Biological Rhythm - it’s the innate mechanism that controls an organism’s body functions on a daily, seasonal, or yearly cycle and is entrained by zeitgebers.

• Free-runing period - a cycle of behavior that continues without external cues, showing the natural rhythm of an organism.

• Period of rhythm - time it takes to complete one cycle of activity

• Zeitgeber - the environmental cue which resets the biological clock

• Entrainment - the process of resetting the biological clock to match external cues

• Phase of shift - how much the activity/rhythm has shifted forward or back during entrainment

Exogenous Rhythm

Exogenous rhythms are rhythms that are externally driven (external timing), for example, the germination of seeds in deserts, and the flowering of cacti when it rains.

These are great survival mechanisms that are environmentally triggered.

Examples that are controlled by environmental stimuli​:

• Light intensity rhythms

• Temperature rhythms

• Pressure rhythms

• Geophysical rhythms

In simple terms:

• Think of this as your body reacting to the environment.

• Example: You feel sleepy when it gets dark because your body responds to sunlight (an external factor).

Endogenous Rhythm

Endogenous rhythms are rhythms that are internally driven (involve internal clock). 

They are great survival mechanisms because they continue in the absence of external environmental factors. (can be adjusted by the environment).

It involves an internal clock like:

• Circadian rhythms

• Tidal rhythms

• Lunar rhythms

• Semilunar rhythms

• Circannual rhythms

In simple terms:

• This is the actual rhythm that results from your biological clock.

• It happens naturally, even without external cues.

• Example: Your sleep-wake cycle is an endogenous rhythm because it follows a natural pattern controlled by the biological clock.

Advantages

1. Enables organisms to anticipate changes in the environment. 

For example:

• Bats spend their “day” in the darkness of a cave. They have no external stimuli to tell them when it's dusk so they need an internal cue to tell them when it's time to begin activity.

• Weta must make their way back to their hiding holes before dawn to avoid predation. 

2. Enables metabolic changes associated with changes in the environment to get underway. 

For example:

• Many crustaceans undergo rhythmic changes in colour due to chromatophore (pigment) in the epidermis. 

Biorhythms

Biological Rhythms are the natural cycle of change in our body's chemicals or functions. 

It acts as an internal master “clock” that coordinates the other clocks in your body.

This internal “clock” is located in the brain. 

Therefore, Biorhythms are the natural, rhythmic cycles our bodies undergo over time. 

It is responsible for regulating various physiological processes and behaviours such as sleep, hormone levels and the body temperature.

In simple terms:

• it is the general body cycle

• these are natural changes in your body over time.

• Example: Your energy levels change throughout the day, your body temperature rises and falls, and hormone levels fluctuate.

Biological Clock

Biological Clock is an innate mechanism that controls the physiological activities of an organism which change on a daily, seasonal, yearly, or other regular cycle. 

The biological clock is entrained (re-set) using environmental stimuli (exogenous rhythms) called zeitgebers.

In simple terms:

• This is the internal system that keeps track of time inside your body.

• It controls endogenous rhythms like sleep, hunger, and hormone release.

• Example: Your biological clock is like a timer in your brain, telling you when to feel sleepy or awake.

Free-running Rhythms

When a rhythm is free-running, its period is similar to that of an external rhythm. Because of this, free-running rhythms are prefixed by “circa” (Latin for about). 

Free-running period is the period of rhythms in constant conditions when external cues are eliminated.

• Circadian rhythms have periods for about a day (dian= day)

• Circatidal rhythms have periods of just over 12 hours. 

• Circalunar rhythms have periods of about a month. 

• Circa-semilunar rhythms have periods of about a fortnight. 

• Circannual rhythms have periods of about a year. 

• Compound rhythms occur when an organism responds to multiple environmental rhythms.

Circadian rhythms in humans:

1. Sleep-awake

2. Body temperature

3. Mitosis in the skin, gut lining, and bone marrow. 

4. Urine output and composition. 

5. Mental concentration, acuteness of hearing, sensitivity to pain, and many drugs.

In simple terms:

• It measures how long your body's cycle actually is without zeitgebers (external cues).

• This is what happens when your endogenous rhythm runs on its own, without any environmental influence.

• Example: If you live in a cave with no light or clocks, your sleep cycle might drift to slightly longer than 24 hours

Zeitgeber

Since free-running rhythms are not synchronous with those of the environment, this means that the internal clock must be entrained (reset) by the environment. 

Zeitgeber is the environmental cue used to reset the clock (time giver in German).

The most common cue in circadian rhythms is sunrise or sunset:

• day-active animals → changes at dawn

• nocturnal creatures → changes at dusk

• circatidal rhythms → the turbulence of the water

In simple terms:

• Think of it like a natural alarm clock that keeps your body in sync with the world

• Example: The presence of sunlight tells your body when to wake up and when to sleep.

Entrainment

Entrainment involves three processes:

• Detection of the external rhythm using some form of zeitgeber

• Transmission of a signal from the receptor to the clock mechanism

• A change in the phase  of the clock

Example:

Zeitgeber = light

Receptor = eyes

Entrainment is necessary for:

1. Adjusting to seasonal changes in the time of dawn/ dusk.

2. Allows migratory animals that make considerable E-W movements to continually update their clocks (otherwise would be out of phase).

Phase Shifting

Phase Shifting is a change (either an advance or a delay) in the phasing of an organism's free-running circadian rhythm, usually in response to an acute stimulus, usually forward or back. 

This is often triggered by light, temperature food, or social factors (potential mates). 

The direction and magnitude of a phase shift are dependent on when in the circadian cycle that environmental cue is applied.

DIFFERENCES

1. Difference between Endogenous Rhythm and Free-Running Period

Endogenous Rhythm (Internal Biological Clock)

• This is your body's built-in rhythm that controls processes like sleep, hunger, and hormone release.

• It does not rely on external cues (like light or temperature).

• Example: If you stay in total darkness, your body still follows a sleep-wake cycle because of its internal clock.

Free-Running Period (Rhythm Without External Cues)

• This is what happens when your endogenous rhythm runs on its own, without any environmental influence.

• It measures how long your body's cycle actually is without zeitgebers (external cues).

• Example: If you live in a cave with no light or clocks, your sleep cycle might drift to slightly longer than 24 hours—this is your free-running period.

Key Difference:

• Endogenous rhythm = The body’s internal rhythm.

• Free-running period = How that rhythm behaves when there are no external cues.

2. Difference between Endogenous Rhythm and Biological Clock

Biological Clock (The System That Controls Rhythms)

• This is the internal system that keeps track of time inside your body.

• It controls endogenous rhythms like sleep, hunger, and hormone release.

• Example: Your biological clock is like a timer in your brain, telling you when to feel sleepy or awake.

Endogenous Rhythm (The Pattern Created by the Biological Clock)

• This is the actual rhythm that results from your biological clock.

• It happens naturally, even without external cues.

• Example: Your sleep-wake cycle is an endogenous rhythm because it follows a natural pattern controlled by the biological clock.

Key Difference:

• Biological clock = The mechanism that controls timing.

• Endogenous rhythm = The pattern of activity created by the biological clock.

3. Difference between Endogenous Rhythm and Biological Rhythm

Biological Rhythm (Broad Category – All Body Cycles)

• This is a general term for all natural cycles in an organism.

• Biological rhythms can be influenced by the environment (external) or controlled by the body (internal).

• Example: The sleep-wake cycle, menstrual cycle, and heartbeat are all biological rhythms.

Endogenous Rhythm (A Type of Biological Rhythm – Internally Driven)

• This is a specific type of biological rhythm that is controlled only by the internal biological clock.

• It does not need environmental cues to continue.

• Example: Even in total darkness, your body still follows a sleep-wake cycle—this is an endogenous rhythm.

Key Difference:

• Biological rhythm = All natural body cycles (both internal and influenced by the environment).

• Endogenous rhythm = A type of biological rhythm that runs only on the internal clock.