NEL
Vol. 21 No. 1
Invited Guest Editorial Full
text pdf
(109kb)
2000;
21:9–15
The
human body circadian: How the biologic clock influences sleep
and emotion by Daniel Cardinali
Reprinted with permission: Ciencia e Cultura 1998; 50:172ñ177
Abstract:
Diurnal, nocturnal or seasonal modes of behavior are not passive
responses to changes in the environment; rather, they are generated
by an endogenous circadian pacemaker, entrained by a few environmental
cues like light-dark cycles. Circadian clock mechanisms involve
periodic gene expression, synchronized by a hierarchically superior
structure located in mammals
in the hypothalamic suprachiasmatic nuclei. Cycles of sleep
and wakefulness are the most conspicuous circadian rhythm. Since
modern humans use artificial light to extend their period of
wakefulness and activity into the evening hours, they adhere
to a short-night sleep schedule with a highly consolidated and
efficient sleep. As shown by studies in artificial long nights,
modern humans may be sleep-deprived. Humans have also increasingly
insulated themselves from the natural cycles of light and darkness.
Still, the human circadian pacemaker has conserved a capacity
to detect seasonal changes in day length. A mood disorder involving
a recurring autumn or winter depression (seasonal affective
disorder, SAD) is related to latitude, with the number of cases
increasing with distance from the equator. SAD is ameliorated
by using brilliant light. In nonseasonal depression, mood typically
fluctuates daily, with improvement over the course of the day,
and various physiological functions exhibit an altered circadian
pattern, suggesting a link with circadian disruption. Treatment
of circadian rhythm disorders, whether precipitated by intrinsic
factors (e.g., sleep disorders, blindness, mental disorders,
aging) or by extrinsic factors (e.g., jet lag, shift work) has
led to the development of a new type of agents called ìchronobiotics,î
among which melatonin is the prototype.
