This document provides an overview of chronobiology, which is the study of biological time and circadian rhythms. Some key points: - The suprachiasmatic nucleus (SCN) acts as the master circadian pacemaker in the brain. It regulates circadian rhythms in physiology and behavior through neuronal and hormonal outputs. - Circadian rhythms are endogenously generated but can be entrained, or reset, by environmental cues called zeitgebers, most importantly the light-dark cycle. - At the molecular level, circadian rhythms are generated by transcriptional feedback loops involving clock genes such as Period and Cryptochrome. - Disruptions to circadian rhythms are implicated in psychiatric

1. CHRONOBIOLOGY
PRESENTER:
DR.S.CHANDANA,
1STYEAR POSTGRADUATE
CHAIR PERSON:
DR.RUFUS ,
ASSISTANT PROFESSOR,
DEPT OF PSYCHIATRY,
ANDHRA MEDICAL COLLEGE

2. SCHEME OF PRESENTATION
• Introduction
• Circadian pacemakers
• Molecular Clockwork
• Resetting the circadian clock
• Retinal photoreceptors
• Extra SCN Zeitgebers
• Sleep and circadian rhythm
• Seasonality and circadian rhythm
• Psychiatric disorders and circadian rhythm
• Circadian rhythm and pharmacotherapy

3. • Chronobiology is the study of biological time
• It is derived from the greek(chronos meaning time and biology is the study or
science of life)
• The rotation of the Earth about its axis imposes a 24 hour cyclicity upon the
biosphere.
• Organisms have evolved to occupy geographical niches that can be defined by
three spacial dimensions and also evolved to occupy temporal niches that are
defined by the fourth dimension of time

4. A broad range of frequencies exist throughout biology ranging from
• millisecond oscillations in ocular field potentials
• 17 year cycle of emergence seen in the periodic cicada(Magicicada spp.)

5. HISTORY OF CHRONOBIOLOGY
TIME PERIOD CONTRIBUTOR DISCOVERY
4th century BC Androsthenes
Ship captain serving Alexander
the Great
Described the diurnal leaf
movements of Tamarind
tree
18th century/1729 Jean-Jacques d’Orteus de Mairan
French scientist
Documented 24hr patterns
in the movement of plant
leaves
18th century/1751 Carl Linnaeus
Swedish botanist
Designed flower clock
using certain species of
flowers
Early 20th century Anthonia kleinhoonte
Erwin bunning
Rhythmicity appears to be
endogenous in nature by
plant studies

6. TIME PERIOD CONTRIBUTOR DISCOVERY
1918 Szymanski Animals are capable of
maintaining the 24 hr
activity patterns in the
absence of environmental
cues
1959 Franz Halberg
Father of American
Chronobiology
Coined the term ‘Circadian’
Auguste forel Circadian rhythms noticed
in the rhythmic feeding
time of bees
1970 Ron Konopka
Seymour benzer
Isolated the first clock
mutant in DROSOPHILA
MELANOGASTER and
mapped the period gene
1994 Joseph Takahasi Discovered the first
mammalian ‘clock gene’
using mice

7. • The most extensively studied and best understood biological rhythms is Circadian
rhythms that have a period of about one day
• A defining feature of circadian rhythms is that they persist in the absence of time
cues and are not simply driven by the 24-hour environmental cycle
• Circadian rhythm can be
1. Diurnal
2. Nocturnal
3. Crepuscular

8. Circadian rhythms in human body:
• Sleep-wake cycle
• Body temperature
• Behavior
• Feeding
• Drinking
• Hormonal levels (e.g: melatonin)

9. ZEITGEBER:
• An environmental agent or event that provides the stimulus for setting or resetting
endogenous circadian clock of an organism
• Derived from german(zeit=time and geber =giver)
• The environmental parameter that most reliably corresponds to the period of
Earth’s rotation is the change in the illuminance associated with day-night cycle

10. Circadian axis of mammals can be divided into three distinct functional components
• A master pacemaker situated in Suprachiasmatic nucleus
• A photoreceptor input to SCN that originates in the eye
• The myriad of rhythmic outputs that provide insight into the clockwork of the
circadian pacemaker.

11. HUMAN SUPRACHIASMATIC NUCLEUS.TOP:NISSIL STAIN OF SECTION THROUGH THE
HUMAN HYPOTHALAMUS.SCN INDICATED BY ARROWS LOCATED DORSAL TO OPTIC
CHIASMA
BOTTOM:AUTORADIOGRAPH OF SAME SECTION ,SPECIFIC BINDING OF
RADIOIODINATED ANALOGUE OF MELATONIN INDICATED BY DARKENING OF THE SCN

12. CIRCADIAN PACEMAKERS
Anatomy:
• SCN is the master oscillator
• Peripheral oscillators are found in wide range of peripheral tissues like kidney ,liver
, lung and other sites in brain
• Neurons of SCN are among the smallest neurons in the entire brain ,they possess
short dendrites
• Every neuron in SCN is immuno-positive for GABA
• SUBDIVISIONS: core-defined by presence of calbindin-positive neurons ,
surrounded by shell

13. AFFERENT PROJECTIONS:
AFFERENT PROJECTIONS NEUROTRANSMITTER FUNCTION
Retino-hypothalamic
tract(primary afferent
input)
Glutamate
(Modulator :PACAP)
Conveying photic
information from retina
to SCN
Ipsilateral intergeniculate
leaflet(subnucleus of
lateral geniculate complex)
to SCN
Secondary indirect
pathway:Retina to IGL to
SCN
NPY Encoding environmental
luminance
Midbrain Raphe to
SCN(5HT1B & 5HT7)
Serotonin Modulates light effect on
SCN function

14. EFFERENT PROJECTIONS:
Suprachiasmatic nucleus
Paraventricular nucleus(autonomic division)
Medial forebrain bundle
Spinal cord (T1 &T2 level)
Cholinergic preganglionic sympathetic neurons
Adrenergic postganglionic sym. neurons within superior cervical ganglion
Pinealocytes
Norepineprine
GABA

15. OUTCOME:
• Increased melatonin synthesis occurs when SCN activity is low
• This anti phasic relationship is established by the GABAergic sign-changing synapse
at PVN
• Melatonin receptors on SCN likely provide feedback mechanism by which the anti
phasic relationship is maintained and possibly reinforced

16. • Exposure to light elicits two distinct effects on daily melatonin profile.
1)light acutely suppresses elevated melatoni levels immediately decreasing them to
baseline levels.
2)light shifts the phase of the circadian rhythm of melatonin synthesis.

17. Second efferent pathway
• SCN PVN(parvicellular neurons) CRH
ACTH adrenal gland(zona fasciculata)
release of cortisol.
• Serum levels of cortisol show strong circadian component,being highest in the
morning hours(6 am to 8 am),just as melatonin approach baseline

18. MOLECULAR CLOCKWORK

19. • Many of these molecular components were initially discovered in the fruitfly,
Drosophila melanogaster,leading to the discovery of mammalian orthologs
• The mammalian circadian clockwork consists of interacting positive and negative
transcriptional and translational feedback loops
• The expression of multiple homologs of Drosophila period gene and cryptochrome
genes are positively regulated by the binding of Clock-Bmal1 heterodimers to E-box
enhancers in the promoters of these genes

20. • The products of Per and Cry genes translocate back into the nucleus and repress
their own transcription which constitutes the negative feedback limb
• Light directly influences the molecular clock via its impact on the expression of
Per2,which has either phase advancing or delaying effects on feedback loop
• Clock-Bmal1 also activates expression of the orphan nuclear receptor gene Rev-
Erba

21. • The gene product of Rev-Erba inturn translocates into the nucleus and represses
transcription of Bmal1 gene through Rev-Erb/Ror response elements in Bmal1
gene promoter
• Bmal1 subsequently heterodimerizes with Clock and again activates expression of
Per,Cry and Rev-Erba genes
• This derepression(activation) of Bmal1 gene ,subsequent heterodimerization with
Clock, and activation of Per,Cry and Rev-Erba genes constitute the Positive
feedback limb

22. Post translational modification
• Phosphorylation of some Per proteins by casein kinase
• Histone acetylation

23. RESETTING OF CIRCADIAN CLOCK
• The mean circadian period generated by human SCN is of about 24.18 hours
• To maintain proper phase relationship of behavioural and physiological process
,circadian clock should be reset in a regular basis within a context of 24 hours a day
• Bilateral removal of eyes-incapable to reset circadian clock

24. • Photic sensitivities of visual and circadian systems are different
VISUAL system CIRCADIAN system
DYNAMIC RANGE 14 Log units 3 Log units
Activation
threshold
Less Higher
Duration of light
stimuli required
less longer
Photoreceptor
input
Layer of rods and
cones
Retinal ganglion
cells
Principal task To construct a
spatio-temporal
representation of
environment
Measurement of
ambient
illumination

25. NOVEL CLASS OF RETINAL PHOTORECEPTORS
• Retinal Ganglion cells are the circadian photoreceptors
• These intrinsically photosensitive cells project directly to SCN and also
IGL,olivary pretectal nuclei
• Contain melanopsin
• Few in number
• Represent 1-2% of all retinal ganglion cells in rodent retina

26. RETINAL GANGLION CELLS IN A FLAT MOUNTED MOUSE RETINA,LABELLED BY INDIRECT
IMMUNOFLORESCENCE WITH AN ANTIBODY AGAINST MELANOPSIN.NOTE THE
PHOTORECEPTIVE NET FORMED BY OVERLAPPING DENDRITIC ARBORS

27. • These cells are less sensitive to light compared to rods and cones of visual system
• They require light stimuli of relatively long duration to be activated
FUNCTION:
• Light induced circadian phase shifting
• knockout study in mice-photoreceptor mediating the residual sensitivity are likely
to be rods and cones

28. EXTRA SCN ZEITGEBERS
• FOOD-has the ability to entrain the circadian rhythm in the absence of the master
clock
• METHAMPHETAMINE-synchronizing the stimulus in the periphery
• EXERCISE-shifts heart rate and clock gene expression rhythms
• These may provide novel treatment targets in efforts to stabilize or shift circadian
rhythms

29. SLEEP AND CIRCADIAN RHYTHMS
• Sleep is the integrated product of 2 oscillatory processes
• 1st process is Sleep homeostat:arises from the accumulation and dissipation of
sleep debt
• 2nd process is governed by circadian clock:controls daily rhythm in sleep propensity
or arousal

32. • Chronotype is dictated almost exclusively by circadian process of sleep regulation
• Morning and evening chronotype loosely reflect advanced and delayed circadian
rhythms relative to environmental light cycle
• Morning larks tend to awaken earlier and experience core body temperature
minimum at earlier clock time relative to those who are labelled as evening types

33. SEASONALITY
• Many organisms are capable of synchronizing physiology to seasonal cycles to
maximize survival
• E.g. precise seasonal cycles in reproduction
• Large mammals-sheep-long gestation period-short day breeders,conceive in fall
,birth occurs in spring
• Hamsters-short gestation period-long day breeders ,conceive in spring ,give birth in
summer
• Circannual rhythms tend to persist in the absence of seasonal cues with
endogenous periods of one year

34. CIRCADIAN RHYTHMS IN PSYCHIATRIC DISORDERS
SEASONAL AFFECTIVE DISORDER:
• Characterised by recurrent major depressive episodes followed by periods of
remission or change to mania or hypomania,that recur on a seasonal basis
• SAD is diagnosed as MDD, bipolar 1 or bipolar 2 disorder with the specifier “with
seasonal pattern” in DSM-5
• The seasonal pattern criteria include

35. 1. Regular temporal relationship between the onset of major depressive episodes
and a particular time of the year
2. Full remissions (or a change to mania or hypomania)also occur at a characteristic
time of the year
3. Two major depressive episodes meeting criteria A and B have occurred in the last
2 years and no nonseasonal episodes have occurred in the same period
4. Seasonal major depressive episodes substantially outnumber the nonseasonal
episodes over the individual’s lifetime

36. • The most prevalent form of SAD has an onset in the late fall and early winter and
remits in the late spring and early summer referred as winter SAD or winter
depression
• Prevalence rate is 4 to 9%
• Women are 4 times as likely to be affected compared to men
• 20% of population have subsyndromal symptoms
• Twin studies identified a heritability of 29 to 43 % for seasonality

37. SYMPTOMS:
• Increase in sleep rather than decrease
• Hyperphagia
• Increase in weight
• Heightened sensitivity to interpersonal rejection
• Leaden feeling in the extremities

38. • The gold standard treatment for winter SAD is light therapy
• A typical prescription for light therapy involves 45 to 90 minutes daily exposure to a
broad spectrum ultraviolet filtered white light source of relatively high
irradiance(5000-10000 lux)
• CBT is equally efficacious for acute treatment and prophylactically to reduce
reccurences

39. NONSEASONAL UNIPOLAR DEPRESSION AND CIRCADIAN
RHYTHMS
• Individuals with depression are more likely to be evening chronotypes and often
have delayed rhythms and reduced peak amplitude of melatonin and core body
temperature
• CLOCK and CRY genes have been found to be associated with depression diagnosis
• Circadian control of sleep is a potential avenue to improve long term outcomes in
depression as sleep improvement is an important marker for lower recurrence
rates

40. • Chronotherapies for the improvement of sleep include Wake therapy , advancing
sleep phase, bright light therapy and melatonin administration
• Administration of agomelatine 5 hours prior to bedtime can advance rhythms and
improve mood.
• The practise of using these therapies with anti-depressant medications shows
promise for achieving rapid response

41. BIPOLAR DISORDER AND CIRCADIAN RHYTHMS
• Circadian abnormalities have been suggested as a potential important etiological
mechanism in the development of BPAD
• CLOCK 19 mice models suggest the importance of circadian modulation of
dopaminergic activity within VTA in BPAD
• Depression is associated with a phase delay and mania associated with phase
advance
• Stabilization of these rhythms may help in preventing or reducing the further
episodes
• Interpersonal Social Rhythm therapy was developed due to the observation that
irregular daily routines resulting in circadian dysregulation precipitates mood
disorders

42. Other psychiatric disorders that may improve in response to light therapy include
• Post partum depression
• Pre-menstrual dysphoric disorder
• Bulimia nervosa
• ADHD
• Disordered sleep due to Traumatic brain injury

43. CIRCADIAN SLEEP DISORDERS
Advanced sleep phase syndrome
• It is a pathological extreme of the morning lark phenotype
• FASPS(autosomal dominant familial form)
• They exhibit striking 4 hour advance of sleep-wake rhythm
• They typically fall asleep at 7.30 pm and spontaneously awaken at 4.30 am appr
• Affected individuals have SNP in PER2 gene

44. Delayed sleep phase syndrome:
• A circadian sleep disorder in which the individual’s internal body clock is delayed
with respect to external day-night cycle
• A length polymorphism in a repeat region of PER3 gene is associated with morning
chronotype while short allele associated with evening chronotype

45. Irregular sleep-wake Disorder:
• Characterized by at least 3 sleep episodes per 24 hour period ,irregularly from
day to day
• Occurs in elderly people with dementia,ASD,traumatic brain injury
Free running/Non-24 hour Sleep-Wake Disorder:
• Circadian sleep disorder in which an individual falls sleep later each day
• Generally the delay is about 1-2 hrs corresponding to a circadian cycle of 25-
26 hrs but some individuals with Non-24 hour exhibit a longer delay
• Chronotherapeutics are often first line of treatment in Circadian rhythm sleep
wake disorders

46. CIRCADIAN DESYNCHRONY
• One’s circadian clock is being desynchronized from local time
• Occurs in jet lag and shift work
SHIFT WORK SCHEDULE:
• When a person works against the natural day-night cycle ,they would suffer from
disruption of circadian rhythm

47. JET LAG:
• There would be a phase difference when a person travels through different time
zones
• In jet lag,the clock is slow to reset so that after time zones have been crossed the
endogenous signals for sleep and wakefulness do not match the local light-dark
cycle
• Symptoms include reduced alertness,day time fatigue,loss of appetite, reduced
cognitive skills and disruption of sleep/wakefullness

48. • SOCIAL JET LAG:
• desynchronization of one’s circadian clock with social environment.
• people who naturally exhibit delayed sleep wake patterns ,able to sleep on
their normal delayed pattern on weekends but are forced to wakeup earlier
during work days

49. MEMORY,AGING AND NEUROCOGNITIVE DISORDERS
• The importance of circadian alignment in normative memory performance is
established by animal studies
• As humans age and normative age related memory problems begin , individuals
experience shortening of circadian period as well as phase advances resulting in
earlier wake up time and bedtime and decrease in the amplitude of most circadian
rhythms

50. • Age related circadian changes are exacerbated in age related neurocognitive
disorders like alzheimers
• The amount of disruption in activity rhythms has also been associated with severity
of neuropathology
• Dysregulated Circadian rhythms have been found to predate the occurrence of
dementia or clinical diagnosis of AD

51. OTHER CIRCADIAN CLOCK ASSOCIATED PATHOLOGIES
• Cancer :Epidemiological studies have shown women working night shifts have
shown elevated risk of breast cancer
• Metabolic syndrome
• Cardiovascular diseases

52. CIRCADIAN RHYTHMS AND PHARMACOTHERAPY
• TCA’s and SSRI’s reduce the elevated nocturnal body temperature in depression
while enhancing the circadian amplitude
• Lithium causes lengthening of circadian period
• Short acting BZD’s(triazolam and brotizolam)have been shown to reduce light
induced expression of clock genes per1 & per2 in SCN.
• Recreational drugs of abuse(MDMA)-serotonin neurotoxin, also affect circadian
system.

53. • A Nobel prize in the field of physiology/medicine was awarded to Dr.Jeffrey
Hall,Dr.Michael Young and Dr.Michael Rosbash in 2017 for their discovery of
molecular mechanisms controlling circadian rhythms
• RECENT STUDIES: Blue –blocking glasses as additive treatment for mania for
improving sleep
• It is an evolvement from dark therapy and builds on the discovery of blue light
sensitive retinal ganglion cells that signals daytime to brain

54. REFERENCES
• Kaplan and sadock’s comprehensive textbook of psychiatry-Tenth edition
• Kaplan and sadock’s synopsis of psychiatry-Eleventh edition
• Shakya P, Roy SN, Deep R. Chronobiology: Discovery of the molecular
mechanisms of circadian rhythm. J Mental Health Hum Behav 2018;23:69-71
• Henriksen TEG, Grønli J, Assmus J, Fasmer OB, Schoeyen H, Leskauskaite I,
Bjorke-Bertheussen J, Ytrehus K, Lund A. Blue-blocking glasses as additive
treatment for mania: Effects on actigraphy-derived sleep parameters. J Sleep
Res. 2020 Oct;29(5):e12984. doi: 10.1111/jsr.12984. Epub 2020 Jan 21. PMID:
31967375.