Rest is when the body repairs and grows. Your session in the gym puts the muscle and nervous system through stress, then when you are cooling down the repair process begins.
The first 20 minutes after your session is the optimal time to refuel by getting some carbohydrate in the form of fruit or starchy vegetables like sweet potato and some protein in the form of animal meat or a quality protein powder.
Taking it to the next level would be to be drinking some water mixed with branched chain amimo acids (BCAA) during and immediately after your workout.
Then within the next 90 minutes having a meal that contains some meat (protein) and some vegetables and fat to give you the minerals and vitamins you need to repair the damage you have just inflicted on yourself.
Now you have refueled its time to rest, get 8 hours (optimally) of good sleep and wake up the next day ready for the next session.
So what happens when you have followed this structure for consecutive days and you wake up the next day feeling:
Unmotivated, irritable, persistent muscle soreness, depressed, loss of appetite, developing injuries
You may be training too often and need a scheduled REST day.
For the majority of people CrossFit programming works best on a 3 days on, one day off 2 days on one day off kind of format.
Great if you have the life schedule that permits this kind of routine but not everybody is so lucky.
3 days on means you train 3 days straight, follow the program. Take the next day off. Then back into the gym for 2 more straight days. Have Sunday as a rest day.
A rest day doesn’t mean you lie on the couch and sleep all day. Rest days can consist of light training at 20-30% max heart rate. Doing extra mobility work with the rollers and lacrosse balls. Walking the dog, swimming, walking, massage sauna.
All of which help to repair the body faster.
There is a difference from needing a rest day of course and just taking one out of laziness. Most people who love CrossFit have the inclination to over train and not take a rest day until their shoulder is dislocated.
Taking your rest day strategically will lead to fewer injuries and greater gains in the gym. Its good for your mental health and for you performance.
Some people are genetically gifted and require fewer rest days as their body repairs and grows quicker than others. But for most of us a strategic rest day is going to lead to a happier more successful athlete with fewer injuries and breaking records sooner.