Some people do, but is it true on average? Maybe not, says Ben Goldacre (via Andrew Sullivan):
Back in 1883
Esquirol commented on the higher incidence of suicide in spring and
early summer. Swinscow showed the same thing with all UK suicides from
1921-1948. So that’s not really winter blues. A study in 2000 looked at all UK suicide data from 1982-96 and found that even this seasonal pattern had pretty much disappeared.
What about elsewhere? A 1974 study on all suicides in North Carolina
(3,672) and admissions to their Veterans Hospital Psychiatry Service
(3,258) from 1965 to 1971 showed no seasonal variation. A 1976 Ontario
study found peaks of suicide and admissions for depression in spring
and autumn. Suicide is highest in Summer, says a paper from Australia in 2003. I’m really not getting this Blue January thing.
Maybe you want data from the general population on mood. A study in 1986
looked at 806 representative males from Finland and found low mood more
common in the summer. Some studies do find higher rates of depressive
symptoms in the winter (Nayyar and Cochrane, 1996; Murase et al.,
1995), but then, some find the opposite results, like a peak in the
spring (Nayham et al., 1994) or summer (Ozaki et al., 1995). One study
from just last month proactively asked 360 patients to rate
their mood regularly, rather than waiting for an event, and found no
relationship, again, between mood and season.
Maybe there are other sources of data you could explore? A paper
looking at GP prescriptions for antidepressants in 1984 found a spring
peak. An earlier paper from 1981 (Williams and Dunn) looks at
prescriptions from 1969-75 and finds peaks in February, May and
from the same year looked at GP consultations for depression and found
peaks in May-to-June and November-to-January (they found similar
results for osteoarthritis, oddly).
Hail Ben Goldacre! Here is my previous post on Ben Goldacre.