I had to cold shower for a January fortnight when we had issues with our gas boiler.

One thing I noticed was that it got me alert much sooner in the morning. Typically I would get up at 7:00am, shower, eat and then head off to work to start at 9:00am, but my brain wouldn't really be in gear till about 9:30am.

With cold showering the brain was in gear far sooner. I felt more willing to get out and get stuff done. Maybe that earlier alertness helps cancel out a desire to pull a sickie?

That being said, the extra alertness wasn't worth the sheer torture of a cold shower.