John Piper, from his book, When I Don't Desire God:
I earnestly recommend that it be in the early morning, unless there are some extenuating circumstances. Entering the day without a serious meeting with God, over his Word and in prayer, is like entering the battle without tending to your weapons. It's like taking a trip without filling the tires with air or the tank without gas. The human heart does not replenish itself with sleep. The body does, but not the heart. The spiritual air leaks from our tires, and the gas is consumed in the day. We replenish our hearts not with sleep, but with the Word of God and prayer (p. 116).