Getting a proper amount of rest is absolutely essential for building your energy resources. If you frequently work far into the night or have a poor sleep, it stands to reason that you may start to feel a little run down. Though everybody is different, most people need at least seven to eight hours of sleep per night in order to function at their best.
If you have been lacking energy, try going to bed earlier at night. If you can wake up feeling well-rested, it will be an indication that you are starting to get an appropriate amount of sleep at night. If you sleep more than eight hours every night but still don't feel energetic, you may actually be getting too much sleep.
Once in while, you are bound to have nights where you don't get an adequate amount of sleep. When your schedule permits you can also consider taking a short sleep during the day, for sometimes taking a nap is the perfect way to recharge your batteries.