Background: Previous studies have suggested that bipolar patients are super
sensitive to light suppression of melatonin and that this may be a trait ma
rker for genetic vulnerability. The present study was an attempt to replica
te and extend this observation. Propranolol hydrochloride effects were comp
ared with light effects because of the documented influence of beta-adrener
gic receptors on melatonin production. Nighttime levels of corticotropin an
d cortisol were also examined as potential trait vulnerability markers.
Methods: Melatonin levels in euthymic bipolar patients (n=29) were tested b
efore and after 500-lux light was administered between 2 and 4 AM and on a
separate night in the dark. Results were compared with those of a group of
patients with unipolar depression (n=24) and with those of a group of non-p
sychiatrically ill control subjects (n=50). Lithium effects and propranolol
effects were tested in subgroups.
Results: No group differences were seen in light suppression among bipolar
patients, unipolar patients, and controls; an analysis of the whole group d
id not reveal differences in propranolol effect, differences in corticotrop
in or cortisol levels, or evidence for a lithium effect. Holo ever, patient
s with bipolar I affective disorder showed the following: (1) significantly
lower melatonin levels on the light night, at baseline and following light
exposure; and (2) a later peak time for melatonin on the dark night.
Conclusions: The general hypothesis of increased light sensitivity in bipol
ar patients was not supported. However, melatonin secretion abnormalities w
ere confirmed in the subgroup with bipolar I disorder. Further assessments
of circadian rhythm disruption as a vulnerability marker in bipolar illness
are indicated.