"You need enough sleep to wake up and not feel tired and fatigued the next morning, " he says.

WSJ: When It's Worth It to Skip a Workout and Sleep

You need to get enough sleep.

WSJ: It Takes Brass to Play the Wagner Tuba | By Barbara Jepson | The Ring of the Nibelung

So unless you are lucky enough to not need the sleep and feel well without it, I would try to exercise in a way that does not cut into your rest.

CNN: Exercise plus sleep may lower cancer risk in women

It's an enzyme called tryptophan which helps you manufacture enough serotonin so that you can sleep when you need to and it helps keep you alert and able to concentrate.

CNN: Beating the stress of business travel

Transportation workers and management need to learn to minimize fatigue, starting with adoption of safer work schedules that provide enough time for sleep each day.

CNN: FAA knew controllers nap, ignored fatigue issue