Sleeping a 15 minute nap is better than 20 ?

Create a habit of going to bed and waking up at the same time each day, even on weekends. This helps anchor your body clock to these times. Resisting the urge for a lie-in can pay dividends in alertness.
If you feel you haven't slept well, resist the urge to sleep in longer than normal; getting up on schedule keeps your body in its normal wake-up routine.

Remember, even after only four hours, the brain has gained many of the important benefits of sleep.

Most of us have a natural dip in alertness between 2 - 4pm.
A 15 minute nap when you're tired can be a very effective way of staying alert throughout the day. Avoid napping for longer than 20 minutes, after which you will enter deep sleep and feel even worse when you wake up.