Anna Giulia Cattaneo, M.D.
DBSM - University of Insubria, Via J-H
Dunant 3 - 21000 Varese.
Changes in sleep quality and rhythms are frequent complaints
in elder subjects: diurnal sleepiness, nocturnal periods of
wakefulness and early awakenings became more frequent as aging
progresses, and cause enhanced intake of hypnotic medications.
Because of sleep-wake rhythm shows a circadian pattern, altered
circadian clock regulation in advanced age has been advocated
as a possible causative mechanism explaining insomnia in this
range of age.
In spite of several clinical findings partially supporting
this hypothesis, the problem seems to be more complex and
involving at least two different pathways, evolved in Vertebrates
and in Mammals.
While in lower animals circadian clock and biochemical changes
linked to the environmental light are linked, in Vertebrates
and in Mammals two distinct functions have been developed,
one mediated by molecules mainly expressed in the retina and
influenced by the light in the environment, the other showing
an intrinsic cyclic activity. These last can be defined as
mechanism whit a near 24 hours-periodicity, determined by
the activity of several regulatory genes which are located
in the bilaterally paired suprachiasmatic nuclei (SCN) of
the hypothalamus. Their expression shows oscillations within
24 hours, persistent even in the absence of environmental
stimuli like light.
Both these mechanisms have been advocated for explaining
sleep disruption and deprivation.
A summary of the main features of these genes in humans beings
and of their interactions will precede the description of
some methods used to study the circadian clock in humans and
results found in aging subjects.
Main circadian cycle regulators in humans and Mammals
1. Gene Clock. Hortolog of the gene found in mouse, its transcript
is a Hypoxia Inducible Factor (HIF), belonging to the basic
helix-loop-helix (bHLH) family of transcription factors and
having a Hat (histone acetyltransferase). Chromosome: 4, 4;
Locus: 4q12. Genetic polymorphism has been related to mental
illness (schizophrenia and bipolarism) and possibly to sleep
2. ARNTL (Bmal1). Its transcript, a basic helix-loop-helix
protein, dimerizes with the transcript of the CLOCK gene,
enhancing the HAT activity. chromosome: 11; Location: 11p15
3. PER. As recalled before, this gene is a hortolog of the
PERIOD (PER) of Drosophila (44% homology) and two point mutations
in the transcript are responsible for two types of a rare
human inherited disease known as familial advanced sleep phase
syndrome (FASPS). Even in Drosophila the activity of this
gene is related to the sleep timing... In humans the gene
encodes a preprotein containing a basic helix-loop-helix (bHLH)
that gives rise for
4. postranscriptional splicing to three 3 transcripts of 4.7,
3.0, and 6.6 kb: only two of them seems to be regulators of
circadian oscillations PER3 being expressed outside the SNC
and apparently deprived of oscillatory properties. Activity:
histone acetylation and deacetylation. Chromosome: 17; Locus:
5. CRY1.The gene encodes for the cryptochrome, a blue-light
photoreceptor, similar to photolyase found in plants. Photolyases
contain flavin adenine dinucleotide (FAD), and a second, variable
chromophore. They mediate photo reactivation, a repair mechanism
that removes UV-induced DNA damage. Class I photolyases include
microbial photolyases and several plant blue-light photoreceptors.
The gene found in humans (and Mammals) is expressed in the
brain, and despite the dependence from the light of several
functions, shows a cyclic activity not dependent from the
light cycle, so belonging to the group of circadian regulators
as defined before. The transcript is located in the mitochondrion.
Chromosome: 12; Location: 12q23-q24.1.
6. MTNR1A. Melatonin is one of at least two recognized markers
of circadian rhythmicity in Mammals. This gene encodes one
of two melatonin receptor, a G-protein integral to the membrane
of neurons in the hypothalamic suprachiasmatic nucleus, where
it should be be involved in circadian rhythm, and in the hypophysial
pars tuberalis, where it mediates the reproductive effects
Chain of events:
1. Clock and Bmal gene activation
2. Heterodimerization CLOCK/Bmal
3. Drives of reported gene PER (histone acetylation and deacetylation)
4. CRY1 and CRY activation (delayed)
5. PER and CRY proteins autorepression.
6. CRY in turn represses CLOCK/BMAL1 transcriptional activity
by an unknown mechanism.
7. MTNR1B product inhibits circadian firing rhythms in the
Molecules expressed in the retina having a light-related
1. CRY2. The transcript of this gene is similar to the class
II photolyases which have been found in higher eukaryotes.
It is highly expressed in the retina, and has been described
as a vitamin A-based opsin, a blue-light photoreceptor necessary
2. MTNR1B. Like MTNR1A, the product of this gene is a transmembrane
G protein able to bind melatonin. However the gene is mainly
expressed in the retina, and minimally in the CNS, suggesting
its action being involved in cycling linked to daylight or
3. PER3. This transcript of gene PERIOD, in contrast with
PER1 and 2, does not show an intrinsic periodicity. Its cycling
is linked to photoperiodicism and timing is similar to that
The activity of circadian rhythm regulators has been documented
in rodents, in which the PER1 gene is mainly expressed during
the nocturnal activity period, PER2 is synchronous with CRY1
and peaks early in the morning, while CLOCK peaks independently,
late in the evening.
In humans, clinical investigations documenting the circadian
rhythms are mainly based on the phases of melatonin and body
In aged people and advance of the circadian clock seems to
be related with proportional advances in sleep time: the aged
go to sleep and wake too early. Similar features have been
described in a rare human inherited disease known as familial
advanced sleep phase syndrome (FASPS) and affecting even young
people, with major disturbance of sleep timing and sleep deprivation.
The disease is supported by point mutations of one of the
genes regulating the circadian clock, and namely PER2. However,
the only finding of sleep time advance is not sufficient to
explain the complexity of sleep disruption found in many older
people, affecting the sleep architecture and the slow-wave
and REM phases. Such kind of alterations are found in PER3
gene polymorphism, and in old subjects submitted to a higher
amount of light during the nocturnal rest, as could be the
case for patients lying in nursing home.
Under this point of view, it seems possible to conclude that
the insomnia in older subjects can be only in part linked
to the circadian clock advance that seems to be the main alteration
characterizing this mechanism during the aging process. A
role of peripheral retinal mechanisms whose activity is dependent
from the light and dark cycle is possibly involved.
Several studies claim that better regulation of environmental
light quality during the night, or light and dark cycles,
can possibly be useful in limiting the disturbance of sleep
in these individuals, at least in part. A reasonable amount
of physical activity, as prudent for the clinical conditions
of the aged patient, can also be of some help.
Pharmacological treatment with non traditional drugs, like
chronobiotics, of the age-related insomnia, need enhanced
knowledges to be considered safe and efficacious.
The applications of mathematical modelling to circadian cycles
could open a new way to pharmacodynamic; with the restraint
that severe scientific control will be applied.