This tip may not work for all - some people just can't fall asleep quickly enough at day time. Give it a try and watch your body reaction. Studies show that 20 minutes of afternoon nap gives more rest to the body than 20 minutes of sleep in the morning. Use this powerful refreshing tool to get energized till the end of the day. Best time to take a nap is midday, otherwise you risk to shift your sleeping pattern.

It's recommended that you sleep for no more than 30 minutes, or you will feel drowsy after waking up. Use an alarm clock until your body gets used to the afternoon nap.

Once you get accustomed to sleep at day time, you will fell that you need less time for the night sleep, and getting up early will become much easier!

Here is couple of links to articles discovering the power of taking a nap (btw, sometimes it called "power nap" :) -
At last, an excuse for the afternoon nap
Power Napping for Increased Productivity, Stress Relief & Health