It is VERY important to rest your body from working out. The reason is muscle recovery. Your muscles need time to repair, and your glycogen stores need replenishing.
The rest needed will depend on they type of exercise you did, the intensity, and your fitness level. A good gauge is how TIRED you feel during your NEXT workout. If you don't have as much 'juice', then you may be over training, or just did not eat enough to re fuel your body.
Eating to recover properly is a tricky one especially if you are trying to lose weight. The best thing for your body after a workout is to eat within one hour after your workout. It just needs to be small. A little carbohydrate mixed with a protein. I am training for endurance so my body needs the glycogen replacement. That comes from complex carbohydrates like whole grains. I can get that in a shake with a bunch of stuff but the easiest and simplest is pasta. As I continue to train my body is learning how to efficiently use the glycogen in my muscles FIRST for energy so I need it. I will naturally start to burn more fat at rest or by doing a less intense exercise because my metabolism will increase. This is the same concept for weight training. The more muscle I have the higher my metabolism and the less chance my body will want to store the food I eat as fat.
So workout and eat properly but make sure you are eating enough to refuel your body....
It's kind of complicated but simple at the same time.
Make sure you are also sleeping enough and as a side note....Sometimes if you are having a really tired day it may be because your period is coming.