Rest days are needed, regardless of intensity from workout you do, whether it's high or low. So it's important to see closer about that.
In workout field rest days can be defined as your rest phases from exercise activity.
This phase can be so important, and most notably for people who do heavy exercise.
For your success in the better fitness, and since body may get injured without you realize.
The rest will be so important for the muscle to recover again.
Rest days can be also used for evaluating the training process, and reflect to see your effort for it actually works or not, and you can actually watch closer about the injury.
So, here see more benefits for the health when you do the activity.
While you're resting, this is the chance to relieve the muscle pain.
The energy that is "kept" within muscle will be recovered too. Surely the muscle can also get exhausted because of exercise.
The over-training situation can also increase the potential of injury, and the situation can be still avoided fortunately.
For mental side, exercise may also affect it, so you may need rest to clear mind first.
Rest days can be practiced one week or more after you stop doing exercise.
When the body shows the certain sign of fatigue, then you can also choose to stop doing your physical activity first.
If you can't do HIIT training in half a hour, for example, where usually you have no problem doing that, then definitely you are exhausted.
Once again, recover your body and mind through the rest day.
DOMS or sore after doing exercise activity should be a common thing. But of course, when it keeps coming then the body gives you the sign to stop doing the exercise.
Another sign when you eat more, and you don't care about your diet anymore.
Apparently body wants to "fix" itself because of the fatigue.
The sign when you are fatigue can be seen in your mood, and not just depressed, but also easy to get offended.
The exhaustion may also cause the imbalance of hormones, hence the situation of mental above.
When your exercise routine feels stuck, or basically does not show any sign of success, depending on goal like the loss of weight, then you must review your whole activity.
Definitely you have hit what's called with "plateau" where body has adapted with only certain exercise method you do.
When you are already exhausted then it'd be easier in becoming injury prone, since you push muscle without enough rest.
Few days for rest should be enough in certain condition, and of course you can balance it according with your training intensity.