DISCUSSION
The circadian rhythm is known to affect inflammation and
immunity. The pathogenesis of conditions such as diabetes,
rheumatoid arthritis (RA), and cardiovascular disease have
been at least partially associated with circadian rhythm dysfunction.
22 For example, the prominence of joint symptoms in
the morning in RA coincides with the surge in proinflammatory
IL-6 in the serum of RA patients.22 Acute cardiac events have
also been noted to be more severe in the morning.22
With regard to inflammation in the skin, such as in psoriasis, it
has been shown that psoriatic mice subjected to sleep deprivation
had increased levels of pro-inflammatory cytokines such as
Kallikrein 5, IL-1, IL-6, IL-12.23 It has also been reported that there
is a higher incidence of psoriasis in American female nurses
that work rotating night shifts versus those that reported no
night shift work.24 This suggests that disruption of the circadian
rhythm plays a vital role in pathology.
In our study, we examined an acute and measurable event,
the erythemal response to UV in the morning versus the late
afternoon. At the molecular level, UV-exposed skin leads to erythema
and causes breaks in DNA integrity, as well as CPDs and
pyrimidine-(6,4)-pyrimidone photoproducts, which both disrupt
the DNA helix.25,26 It has been demonstrated that the circadian
clock regulates DNA replication, and NER in an antiphase circadian
rhythmicity.10
Comparing the human data from our study with the previously
published mouse data,10 it is interesting that the same trend of
circadian pattern of erythema was observed. In both our study
and the previous mouse study, AM UV exposure was more
