As merry as the day is long? Shakespeare on seasonal mood ...

Easy to over-interpret, however when Beatrice says in Much Ado about

Winter - North of Scotland

Nothing she is " ... As merry as the day is long", interesting to consider: was this a throw-away line, a mask for her true mood, or the mirror of "... As sad as the day is short"?  We also have young prince Mamillius saying "... a sad tale's best for winter." [Winter's Tale. II.i.25]

Hippocratic writings [On Airs, Waters, and Places: ~400 BCE] recognize the potential for seasonal variation to influence patterns of disease in general. And not a new idea that people are lower in spirits in winter months: the Roman writer Jordanes refers to winter depression in his history of the Goths [Getica: 551 AD]. Describing depression as 'the blues' dates from 1741 and 'winter blues' [la dépression hivernale] describes what many people may experience. 

However SAD syndrome (Seasonal Affective Disorder) was only formally reported as such and named in 1984 by Rosenthal and colleages at the National Institute of Mental Health in the USA. Rosenthal was initially motivated by wishing to discover the cause of his own experience of depression during the dark northern US winter. His key contribution was to suggest that reduced exposure to light is a major factor causing SAD and that exposure to more light during the day may reverse features of SAD. 
 

Of note, Much Ado (as Love's Labour's Won) is currently paired by the RSC in Stratford with the bleaker Love's Labour's Lost with its unfulfilled ending, and the poem "Winter", "When icicles hang by the wall". The only "merry note" appears to be from an owl. Chilled Dick the shepherd is "blowing his nail", greasy Joan, has "... nose, red and raw...", and the "Parson (is) drowned out by coughing flock".

Shakespeare often used winter as a metaphor for hard times and troubled characters, from the direct contrast from the opening of King Richard III

"Now is the winter of our discontent, made glorious summer ..."

to Sonnet 5

"For never-resting time leads summer on
To hideous winter and confounds him there;
Sap cheque'd with frost and lusty leaves quite gone,
Beauty o'ersnow'd and bareness every where."

There is inevitable speculation about the extent to which Shakespeare was revealing on stage aspects of his own depression, or reflecting human experience, and the influence of uncertainties of life at his time.

For the present day, there are now also biochemical hypotheses for the cause of SAD - infections are common in winter and the inflammatory chemicals (cytokines) released by the body's defence cells to fight infection may have the unwanted effect of depressing mood, through toxic actions on brain cells. This may both provide new causes for depression as well as opportunities for treatment, through blocking these harmful chemicals.

Folk wisdom and health

Shakespeare and the history of heartbreak  The Lancet
 

Sadness and the four humours in Shakespeare

Shakespeare and depression