"Meditation is particularly important in the morning. We do not
realize how the mind becomes conditioned during the period from 4am
onwards. It is that time of the night when we are neither fully awake
nor fully asleep when the mind becomes very vulnerable to things we
experienced the day before, or perhaps we never experienced at all.
But these negative vibrations are there and the mind tends to pick
them up and they enter the dream state. By the time we wake up, our
mind is already conditioned by these negative vibrations, and so it
becomes very difficult to meditate because with a mind charged with
such vibrations it is difficult to concentrate on something else.

Getting up early means that we get up with a fresh, clear, free mind.
Then we are able to fill the mind with positive vibrations, positive
thoughts, and through this we will find that the mind gets nourished
by this positive thinking, gets strengthened by it, and it helps to
keep the mind steady. Therefore, early morning meditation is very
important. At 4 o'clock in the morning, before getting up, we can sit
for one, two or three hours in meditation."
http://www.spiritual.com.au/articles/meditation/interviewmeditation_Nirliptanand\
a.htm
From: An interview with Swami Nirliptananda about the method and
purpose of meditation