Class Worksheet: Biological rhythms
Many biological processes take place in cycles of activity that correspond to changes in
the environment, such as the cycles of day and night, or the seasons. Like other
organisms, humans have evolved mechanisms that allow us to increase and decrease our
biological activity in response to these natural cues.
Overview: Types of biological rhythms
Length Example(s)
Ultradian Shorter than a day, less than 24 hours Stages of sleep, hunger
Circadian About a day, 24 hours The sleep-wake cycle
Infradian Longer than a day, more than 24 Menstruation, SAD
hours
1. Circadian: The Sleep-Wake Cycle
The sleep-wake cycle is a circadian rhythm which determines when we sleep and wake
during the 24-hour day. Most of us sleep for 7-8 hours at night, but there is a lot of
individual variation. There are two mechanisms that control the sleep-wake cycle:
1. Endogenous pacemakers (EPs) are internal biological mechanisms that
spontaneously produce timed rhythmic activity.
2. Exogenous Zeitgebers (EZs) are external factors that ‘tune’ the cycle into
environmental factors.
The main EP for the sleep-wake cycle is the suprachiasmatic nucleus (SCN)
- SCN is a tiny region of the hypothalamus which acts as a ‘biological clock’
- SCN production of proteins rises and falls over an approximately 24-hour cycle.
- This rhythmic activity is free-running and genetically determined, but it can be
influenced by external factors (EZs) such as light.
Light is the main exogenous zeitgeber (German for ‘time-giver’)
- Light is able to affect the functioning of the biological clock, the SCN.
- Falling light levels are detected by a protein called melanopsin in the retina, which
passes this information to the SCN through the optic nerve.
- The SCN instructs the pineal gland to increase the conversion of serotonin into
melatonin, a hormone which promotes sleepiness.
- Other Zeitgebers include meal times, social activities and clocks.
RESEARCH + EVALUATION
SIFFRE
(+)SIFFRE > 2-6 MONTHS NO DAYLIGHT> SLEEP-WAKE CYCLE ADJUSTED TO 25 HOUR CYCLE >
REGULAR SLEEP-WAKE CYCLE > FREE RUNNING CIRCADIAN RHYTHM > EVIDENCE THAT
BIORHYTHMS CAN PERSIST WITHOUT EXTERNAL CUES
, Siffre (a cave explorer) spent long periods of time (2 months – 6months) living underground with no
daylight, no clocks, no radio, simply waking and sleeping when he wanted. Over two months, he found
that his sleep-wake cycle adjusted to about a 25-hour cycle. He tested these patterns by removing time
cues including watches and daylight. Importantly he found that he did have regular sleep-wake cycles,
supporting the existence of a free-running circadian rhythm. This case study provides evidence that
biorhythms can persist without external cues.
DEMAND CHARACTERISTICS
(-)DEMAND CHARACTERISTICS -> WILLINGNESS -> BUT HIGH IN RELIABILITY, SIMILAR
RESULTS PRODUCED -> ASCHOFF AND WEVER PLACED PTPS IN UNDERGROUND WW1 BUNKER
FOR 4 WEEKS -> NO ENVI OR SOC TIME CUES -> ALL BUT 1 FREE RUNNING CYCLE 24/25
HOURS -> THE ONE WAS 29 HOURS
This research support the role of the endogenous body clock being on a 24hr cycle. It could also be
criticised for demand characteristics, for example, Siffre’s willingness to be isolated for so long.
However, the studies have proven to be high in reliability as many have been able to produce similar
results. Aschoff and Wever supported this cycle duration by placing participants in an underground WWII
bunker for 4 weeks, with no environmental or social time cues. They found that in all but 1 (whose
sleep-wake cycle was 29hours) the free-running cycle persisted and participants had a sleep-wake
cycle of 24/25 hours.
FOLKARD ENDOGENOUS PACEMAKERS VS EXOGENOUS ZEITGEBERS
FOLKARD > ENDOGENOUS PACEMAKERS WERE STRONGER INFLUENCING CIRCADIAN
RHYTHMS THAN EXOGENOUS ZEITGEBERS > 12 PEOPLE DARK CAVE 3 WEEKS > BED AT
11:45PM -> CLOCKS SPED UP > 22 HOUR DAY > ONLY ONE PP ADJUSTED TO NEW REGIME >
SUGGESTS STRONG FREE-RUNNING CIRCADIAN RHYTHM IS NOT CONTROLLED BY EZ
Folkard et al (1985) showed endogenous pacemakers were stronger influencing circadian rhythms that
exogenous zeitgebers. They studied a group of 12 people who lived in a dark cave for 3 weeks, going to
bed when the clock said 11:45 pm and waking when it said 7:45 am. The researchers gradually sped up the
clocks (unbeknown to the pps) to a 22-hour day. Only one ptp comfortably adjusted to the new regime,
suggesting that a strong free-running circadian rhythm is not controlled by exogenous zeitgebers.
DIFFICULT TO MAKE GENERALISATIONS
GENERALISATIONS ARE DIFFICULT TO MAKE -> SMALL GROUPS OF PTPS -> FOLKARD 12,
SIFFRE ALONE -> PTPS NOT REPRESENTATIVE OF WIDER POPULATION ->
One limitation of biological rhythms research is that generalisations are difficult to make. Studies of the
sleep-wake cycle often use small groups of participants (eg, Aschoff &Weaver, or even single individuals such
as Siffre). Participants may not be representative of the wider population and this limits making
meaningful generalisations. Siffre observed that his internal clock ticked much more slowly at 60yrs old than
when he was younger. This suggest that even when the same person is involved there are factors that may
prevent general conclusions being made.
Many biological processes take place in cycles of activity that correspond to changes in
the environment, such as the cycles of day and night, or the seasons. Like other
organisms, humans have evolved mechanisms that allow us to increase and decrease our
biological activity in response to these natural cues.
Overview: Types of biological rhythms
Length Example(s)
Ultradian Shorter than a day, less than 24 hours Stages of sleep, hunger
Circadian About a day, 24 hours The sleep-wake cycle
Infradian Longer than a day, more than 24 Menstruation, SAD
hours
1. Circadian: The Sleep-Wake Cycle
The sleep-wake cycle is a circadian rhythm which determines when we sleep and wake
during the 24-hour day. Most of us sleep for 7-8 hours at night, but there is a lot of
individual variation. There are two mechanisms that control the sleep-wake cycle:
1. Endogenous pacemakers (EPs) are internal biological mechanisms that
spontaneously produce timed rhythmic activity.
2. Exogenous Zeitgebers (EZs) are external factors that ‘tune’ the cycle into
environmental factors.
The main EP for the sleep-wake cycle is the suprachiasmatic nucleus (SCN)
- SCN is a tiny region of the hypothalamus which acts as a ‘biological clock’
- SCN production of proteins rises and falls over an approximately 24-hour cycle.
- This rhythmic activity is free-running and genetically determined, but it can be
influenced by external factors (EZs) such as light.
Light is the main exogenous zeitgeber (German for ‘time-giver’)
- Light is able to affect the functioning of the biological clock, the SCN.
- Falling light levels are detected by a protein called melanopsin in the retina, which
passes this information to the SCN through the optic nerve.
- The SCN instructs the pineal gland to increase the conversion of serotonin into
melatonin, a hormone which promotes sleepiness.
- Other Zeitgebers include meal times, social activities and clocks.
RESEARCH + EVALUATION
SIFFRE
(+)SIFFRE > 2-6 MONTHS NO DAYLIGHT> SLEEP-WAKE CYCLE ADJUSTED TO 25 HOUR CYCLE >
REGULAR SLEEP-WAKE CYCLE > FREE RUNNING CIRCADIAN RHYTHM > EVIDENCE THAT
BIORHYTHMS CAN PERSIST WITHOUT EXTERNAL CUES
, Siffre (a cave explorer) spent long periods of time (2 months – 6months) living underground with no
daylight, no clocks, no radio, simply waking and sleeping when he wanted. Over two months, he found
that his sleep-wake cycle adjusted to about a 25-hour cycle. He tested these patterns by removing time
cues including watches and daylight. Importantly he found that he did have regular sleep-wake cycles,
supporting the existence of a free-running circadian rhythm. This case study provides evidence that
biorhythms can persist without external cues.
DEMAND CHARACTERISTICS
(-)DEMAND CHARACTERISTICS -> WILLINGNESS -> BUT HIGH IN RELIABILITY, SIMILAR
RESULTS PRODUCED -> ASCHOFF AND WEVER PLACED PTPS IN UNDERGROUND WW1 BUNKER
FOR 4 WEEKS -> NO ENVI OR SOC TIME CUES -> ALL BUT 1 FREE RUNNING CYCLE 24/25
HOURS -> THE ONE WAS 29 HOURS
This research support the role of the endogenous body clock being on a 24hr cycle. It could also be
criticised for demand characteristics, for example, Siffre’s willingness to be isolated for so long.
However, the studies have proven to be high in reliability as many have been able to produce similar
results. Aschoff and Wever supported this cycle duration by placing participants in an underground WWII
bunker for 4 weeks, with no environmental or social time cues. They found that in all but 1 (whose
sleep-wake cycle was 29hours) the free-running cycle persisted and participants had a sleep-wake
cycle of 24/25 hours.
FOLKARD ENDOGENOUS PACEMAKERS VS EXOGENOUS ZEITGEBERS
FOLKARD > ENDOGENOUS PACEMAKERS WERE STRONGER INFLUENCING CIRCADIAN
RHYTHMS THAN EXOGENOUS ZEITGEBERS > 12 PEOPLE DARK CAVE 3 WEEKS > BED AT
11:45PM -> CLOCKS SPED UP > 22 HOUR DAY > ONLY ONE PP ADJUSTED TO NEW REGIME >
SUGGESTS STRONG FREE-RUNNING CIRCADIAN RHYTHM IS NOT CONTROLLED BY EZ
Folkard et al (1985) showed endogenous pacemakers were stronger influencing circadian rhythms that
exogenous zeitgebers. They studied a group of 12 people who lived in a dark cave for 3 weeks, going to
bed when the clock said 11:45 pm and waking when it said 7:45 am. The researchers gradually sped up the
clocks (unbeknown to the pps) to a 22-hour day. Only one ptp comfortably adjusted to the new regime,
suggesting that a strong free-running circadian rhythm is not controlled by exogenous zeitgebers.
DIFFICULT TO MAKE GENERALISATIONS
GENERALISATIONS ARE DIFFICULT TO MAKE -> SMALL GROUPS OF PTPS -> FOLKARD 12,
SIFFRE ALONE -> PTPS NOT REPRESENTATIVE OF WIDER POPULATION ->
One limitation of biological rhythms research is that generalisations are difficult to make. Studies of the
sleep-wake cycle often use small groups of participants (eg, Aschoff &Weaver, or even single individuals such
as Siffre). Participants may not be representative of the wider population and this limits making
meaningful generalisations. Siffre observed that his internal clock ticked much more slowly at 60yrs old than
when he was younger. This suggest that even when the same person is involved there are factors that may
prevent general conclusions being made.
