Resting properly is just as important as training. To improve as a cycler, the process is simple: training, fuel (food and drink) and recovery, repeated in appropriate amounts at appropriate times. If any of these ingredients within the cycle are missing or inadequate you cannot expect to improve as rapidly or safely as you might otherwise.
Fit rest in with your daily and weekly commitments. If you are always tired after a week of work, make Friday a rest day to leave you fresh for training at the weekend. Listen to your body. If y6ou slept badly, it could be that you are over-tired or stressed – both are a danger to you as a rider – and an easy or rest day should follow.
If you lack energy before you even start training, an easy week which incorporates a couple of rest days in a row could really help you get back on the saddle. You also need to eat and sleep well though or the rest will be a waste of time.