14.4 - Circadian Sleep Cycles

Question 1

Is the sleep-wake cycle the only example of a circadian rhythm?

Answer 1

No, most physiological, biochemical and behavioural process is at least a little circadian

Question 2

what is the primary thing that our bodies are adjusting to in terms of our circadian rhythm?

Answer 2

the light and dark cycle

Question 3

what do we call environmental factors that influence circadian rhythms?

Answer 3

Zeitgebers, which entrain or control the timing go circadian rhythms

Question 4

can we alter circadian cycles by manipulating zeitgebers?

Answer 4

yes, in lab settings we can change circadian cycles to a 23 hour day by alternating 11.5 periods of dark and light

Question 5

Do we develop circadian cycles in the absence of the light-dark cycle?

Answer 5

Yes, we can, for instance hamsters living in perpetual darkness or light can be entrained by daily bouts of social interaction, boarding, eating or exercise.

Question 6

why do we use hamsters to study circadian rhythms so often?

Answer 6

bc their rhythms are extreme.y clear

Question 7

What happens to circadian rhythms in the complete absence of any zeitgebers?
- what do we call these?

Answer 7

humans and other animals maintain all their circadian rhythms, which are called free running rhythms in this context, their durationg being termed a free running period

Question 8

what are the primary features of free running periods? (4)

Answer 8

1. vary in length between indiviauls
2. stable within individuals
3. usually longer than 24 hours (around 24.2)
4. extremely regular

Question 9

what did the study of free running rhythms cause us to suspect

Answer 9

that there is some internal biological clock that habitually runs a little slow unless entrained by time related cues in the environemtn

Question 10

what does the regularity of free running sleep wake cycles suggest?

Answer 10

that circadian rights are the primary regulators of sleep, not recuperation or cognitive/physiological expenditure

Question 11

Are free running circadian cycles learned?

Answer 11

No, even rats born and raised in unchanging lab environments display regular free running sleep wake cycles that are slightly longer than 24 hrs

Question 12

Describe internal desynchronization

Answer 12

1. some species display body temp circadian rythhims (temp falls and night and rises in the morning)
2. if housed in constant lab environments, sleep wake and body temp cycles can break away from one another

Question 13

give a human example of internal desynchronization

Answer 13

human volunteer with initial free running sleep wake and body temp rhythms of 25.7 hours

• increase of sleep wake to 33.4 Horus
• decrease in BT to 25.1 hours

Question 14

what does intneral desynchronization suggest? (2)

Answer 14

1. there may be more than one circadian thing mechanism,

2. sleep is not causally related to the decreases in body temp normally associated with it

Question 15

what do instances of free running volunteers staying awake longer show us?

Answer 15

that recuperation theories cannot be true, bc they still tend to wake at the same time, getting less sleep but remaining consistent
(more wakefulness per cycle, less time for sleep)

Question 16

what are the two disruptors of circadian rhythmicity in modern societies?

Answer 16

1. jet lag

2. shift work

Question 17

explain jet lag in terms of zeitgebers

Answer 17

• zeitgebers controlling phases of various circadian rhythms are accelerated during east bound flights (phase advances)
• decelerated during west bound flights (phase delays)

Question 18

explain shift work in terms of zeitgebers

Answer 18

they remain the same, but workers must adjust her sleep wake cycles in order to meet the demands of the work schedule

Question 19

what do both jet lag and shift work produce in terms of symptoms

Answer 19

1. sleep disturbances
2. fatigue
3. genreal malaise
4. physical ad cognitive dysfunction

Question 20

How long do the effects of phase advances tend to take?

Answer 20

upwards of 10 days to adjust to 10 hour phase advances

Question 21

What two behavioural approaches have been proposed to reduce the effects of jet lag?

Answer 21

1. gradually shifting the sleep-wake cycle in the days prior to the flight
2. administering treatments after the flight that promote the shift in circadian rhythm, for instance melatonin

Question 22

what have companies implying shift workers done to improve productivity?

Answer 22

1. schedule phase delays rather than advances as much as possible (easier to stay awake longer and get up later than it is to go to bed earlier and get up earlier)

Question 23

what is the circadian clock?

Answer 23

the bodies’ internal timing mechanism

Question 24

what was the first large breakthrough in the search for the neural basis of the circadian clock?

Answer 24

The discovery that large medial hypothalamic lesions disrupt circadian cycles of eating, drinking and activity in rats

Question 25

Specific lesions to what area of the medial hypothalamus were shown to disrupt various circadian cycles, including sleep wake cycles?

Answer 25

the suprachiasmatic nuclei of the medial hyp thal

Question 26

what exactly do suphrachiasmatic nuclei lesions induce?

Answer 26

abolish the circadian periodicity of sleep cycles, not the amount of time animals sleep

Question 27

what is the non-experimental support for the role of the suprachiasmatic nuclei in circadian rhythms?

Answer 27

Nuclei display circadian cycles of electrical, metabolic, and biochemical activity that can be entrained by the light-dark cycle

Question 28

Explain the Ralph and colleagues 1990 study on the SCN and its implications

Answer 28

1. removed the SCN from the fetuses of a strain of mutant hamsters with short free running sleep wake cycles
2. implanted these SCN into normal adult hamsters whose typical free running sleep wake cycle was abolished by SCN lesions
3. Transplanted SCN restored the free running sleep wale cycles of the recipients to the 20 hour (short) cycle of the mutant hamsters.
   - transplant in the other direction and the opposite effect
   - Proves conclusively that the SCN is the locus of most circadian rythms

Question 29

what three lines of evidence indicate that the SCN is not the only circadian clock in mammals?

Answer 29

1. bilateral SCN lesions have been shown to leave some circadian rhythms in tact what abolishing others
2. bilateral SCN lesions dont eliminate the ability of all environmental stimuli to entrain circadian rhythms
3. similar to SCN neurons, cells from other body parts often display free running circadian cycles of activity when maintained in tissue culture

Question 30

what is an example of the fact that bilateral SCN lesions dont eliminate the ability of all environmental stimuli to entrain circadian rhythms

Answer 30

SCN lesions do block entrainment by light, but not by regular food or water availability

Question 31

How do the SCN control circadian rhythms?

Answer 31

The timing mechanisms of the SCN depend on the firing patterns of SCN neurons, which tend to inactive at night, begin to fire at dawn, and fire at a slow and steady pace all day.

Question 32

What was the first place that researchers looked to study how light-dark cycles entrained sleep-wake cycles?

Answer 32

The eye, trying to identify and track the specific neurons that left the eyes and carried the info about light and dark tha entrained the biological clock

Question 33

what effect did cutting the optic nerve before it reached the optic chiasm have on light entrainment?

Answer 33

eliminated its ability to entrain circadian rhythms

Question 34

what effect did cutting the optic tracts after they left the optic chasm have on light entrainment

Answer 34

no effect

Question 35

what do the findings surrounding the location of optic nerve lesions and their influence on light entrainment imply?

Answer 35

that visual axons critical for the entrainment of circadian rhythms branch off from the optic nerve in the vicinity of the optic chiasm

Question 36

what did the finding that visual axons critical for the entrainment of circadian rhythms branch off from the optic nerve in the vicinity of the optic chiasm lead to?

Answer 36

the discovery of the retinohypthalamic tracts, which leave the optic chimes and project to the SCN

Question 37

what is surprising about the influence of light dark cyc/les on the retinohypothalmic tracts and circadian rhythms

Answer 37

rods and cones are uneccesary - rather, photoreceptors have been found on retinal ganglion neurons that have unique functional properties

Question 38

what are the unique functional properties of retinal ganglion cells? explain their evolutionary history

Answer 38

evolution cased these photoreceptors to loose the ability to respond quickly and briefly to rapid changes in light in favour of the ability to response consistently to slowly changing levels of background illumination

Question 39

what is the photopigment of retinal ganglion photoreceptors?

Answer 39

melanopsin

Question 40

what was the important breakthrough that led to the study of the genetics of circadian rythms?

Answer 40

some hamsters had abnormally short 20 hour free running circadian rhythms p experiments showed the abnormality was a result of a genetic mutation and the mutated gene was named tau

Question 41

what was the first circadian gene tohave its molecular structure characterized?

Answer 41

‘clock’ which was discovered in mice

Question 42

what three discoveries followed from the discovery of circadian genes?

Answer 42

1. same or similar circ. genes have been found in many species of different evolutionary ages, so these genes must have evolved early in evolutionary history and been conserved.
2. key mechanism of circ. genes is gene expression, or the transcription of proteins by the circadian genes displays a circadian cycle
3. proved to be a more direct method of exploring the circadian timing capacities of parts of the body other than the SCN - molecular circadian timing mechanisms similar to these in the SCN exist in most cells ot the body. Most cellular clocks are regulated by neural and hormonal signals from the SCN.