In the muscles of humans and other animals are present in circadian rhythms influencing metabolism and power of muscle contractions. That is why during the day the muscle force can vary greatly.
Scientists have found that morning and lunch is the best time for training. During this period the muscles have a maximum strength and grow best. Scientists explain this by the rapid ingress of oxygen into the fabric.
In the experiment, the scientists watched the force of the contractions in laboratory mice. Taken into account the fact that the mouse is a typical nocturnal animals. In the daytime the muscles of animals worked much worse than at night.
In humans there are similar cycles, but flowing the opposite way: the muscular metabolism is accelerated in daytime and slow at night. Therefore, to postpone exercise in the middle of the night is not worth it.