Great time of year for an ocean dip, or a walk outside

I try to let the garden rest over the festive season. There may be a leek or a frost-sweetened parsnip to pull for today’s winter-warmer soup or a displaced birdfeeder to hang back up after last night’s bluster but I have pretty much trained myself to look at the garden this end of the year and not be out trampling and compacting it. It is healthier for it, but is the time inside healthier for us? The cold weather at this time of year can have an impact on our general health and also trigger underlying conditions. So, should we avoid it at all costs?
Personally, I like to take a few bracing walks in December and January and even the odd winter dip in ocean - as a short-term exposure to cold weather actually activities your immune system. It's said your system perceives the shock of the chill as a threat and so hits the alarm bell for more natural killer cells and other immune cells to prepare to deploy.