I thought I recognised the text - it's from 'A Study in Scarlet' - the first of the series of Sherlock Holmes stories, which gives an idea of the time the story was written and set in.
The paragraph in question describes Watson's first impressions of Holmes:
Holmes was certainly not a difficult man to live with. He was quiet in
his ways, and his habits were regular. It was rare for him to be up
after ten at night, and he had invariably breakfasted and gone out
before I rose in the morning. Sometimes he spent his day at the
chemical laboratory, sometimes in the dissecting-rooms, and
occasionally in long walks, which appeared to take him into the lowest
portions of the city. Nothing could exceed his energy when the working
fit was upon him; but now and again a reaction would seize him, and
for days on end he would lie upon the sofa in the sitting-room, hardly
uttering a word or moving a muscle from morning to night.
Although the modern day definition of reaction is more familiar to us - to do something in response to a stimulus of some sort, a somewhat archaic definition of the word is:
depression or exhaustion due to excessive exertion or stimulation
... which fits the scenario described above. Holmes would work tirelessly, apparently to the point of overexertion, and would then fall into a depression for a few days.
