Your target heart rate is depending on your age and the intesity you are looking for.
You can do it yourself by figuring out your max heart rate on your own, but that requires you to really exert yourself. I would warm up with about half a mile of jogging. Then do 5 x 100m with 30 seconds rest in between.. after each one check your pulse, you should keep going up and up. Then do one final 200m ALL OUT.. literally go until your legs are numb and can't move.. if you can't finish the whole 200m fine, but check your pulse right away.. it should feel like your heart is beating out of your chest. That is your max heart rate.
Now when you do your cardio.. you need to get your heartrate up to about 80% of that MAX heart rate.
Also I would do 30 mins, not 20. It takes about 20 mins to burn off all the sugar out of your blood stream and start tapping into your fat stores.
The sprints at night wont burn fat, but could help speed up your metabolism which would in turn burn fat. BUT it may make it hard for you to fall asleep at night.
Brian