Suggested Reading: Mark 6:35-50
There is a tenth season episode of Stargate SG-1 where the team encounters a parasite that stimulates the parts of the brain that induce sleep. The parasite fed off of the chemicals the body produces during sleep. Due to the parasite's nature, once a victim fell asleep, they never woke up, so the team had to struggle to stay awake over a course of several days while a parasite was stimulating their brain to make them need to sleep. They took stimulants, drank coffee and tag-teamed each other trying to stay awake. One of the most impressive things about the episode was how sleepy each of the actors managed to appear.
We've all been really tired at some point or another and the longer you go, the more you just want to sleep. You don't want to eat. You don't want to talk. You just want to sleep. Mark chapter 6 shows the disciples in just such a state. They had all just returned from their missionary training adventures and were reporting to Jesus but the crowds were so great (apparently their missions had been successes) that they couldn't even eat or rest. So Jesus had them get in a boat and sail a distance away to avoid the crowds.
When they reached the other shore, word had already spread and crowds were waiting on them. Jesus ended up teaching them so long that the disciples suggested he send them away before they fainted from hunger. Instead, we got the miracle of the feeding of the five thousand. When that was done, Jesus sent the disciples away in the boat to rest while he dismissed the crowds. Before returning to the disciples, Jesus spent some time in prayer on a mountain. Then, "he saw the disciples straining at the oars, because the wind was against them. About the fourth watch of the night he went out to them, walking on the lake. He was about to pass by them, but when they saw him walking on the lake, they thought he was a ghost" (Mark 6:48-49, HCSB). The fourth watch was between 3:00 & 6:00 am.
So these disciples, who were already exhausted, spent the entire night rowing, battling the wind. But that moment, when they were exhausted, weak, and at the very end of their strength, is when they saw Jesus walking on the water and when, according to Matthew's account, Peter actually got to walk on water himself (Matthew 14).
You may be very exhausted right now. You may be at the end of your rope with no more energy to do anything. You may be on the point of giving up because you don't have the strength to do anything but quit. But when we are at our weakest is when God tends to do big things. No matter how tired you are, hang in there. You never know when God is going to show you something miraculous.