The body is finely balanced, and grows accustomed to routine. Most of us have routined and predictable lives. In my case, I exercise a certain amount, I eat certain foods, go to bed and wake up a certain times, in short, a fairly predictable routine. And the body grows accustomed to this. A sudden change, and the whole chemical balance is thrown off and the result, you get sick.
A holiday is a sudden change. Suddenly you can be in a different time zone, eating different foods, and because it is a holiday, you stop exercising. A shock like this weakens the immune system. I’m notorious for getting sick when I travel too far. I travelled to Las Vegas recently. I was determined to not get sick. I planned on eating well, using the hotel gym, and even more conveniently, using some fitness apps on my phone that don’t even require me to leave the hotel room. The plan was to deviate as little as possible from my “normal”.
Sadly, I did get sick. By the time my head hit the pillow in my hotel room, 24 and half hours after I’d woken up, I had a sore throat, headcold, and coldsore. It seems my body is more delicately balanced than most. I had packed plenty of medication knowing this to be the case.
Even if you’re not as fragile as I apparently am, lots of people get sick on holidays, or around Christmas because of the break in your routine. Your body adapts to your routine, change it, and the shock can make you sick. This applies to micro as well as macro changes. For no reason in particular, I’ve always combined cheat day with rest day, until recently I heard bodybuilder Dennis James question why anyone would do this, its a shock to the system. Cheat day and rest day should not be the same day to minimise the shock.
The lesson, any changes to your routine should be small and gradual.