When you're tired, napping increases your alertness for the rest of the day.

Some people nap easily; others can't seem to nap at all. You already know into which camp you fall. The best nap time is between 2 and 3 p.m. Any later and your nap may be too deep, interfering with your nightly sleep. 

Naps, however helpful, are not a good substitute for regularly getting the right amount of sleep. It’s best not to use naps to catch up on sleep if you short-change yourself nightly. 

Your quality of sleep will be much higher and the immediate benefits more apparent if you nap in a bed or cot as opposed to a chair.  Although everyone feels a little groggy for a few minutes after a nap, this gradually subsides.  Short naps are more productive than long naps.  A short nap will leave you refreshed; a long nap may interfere with your sleep that evening.   Naps of 20 minutes or less usually help avoid REM sleep, a stage where you're likely to wake up groggy and stay that way.

To derive the most from your nap time, safeguard the nap area before you nod off by making sure that phones, fax machines, or other gadgets will not disturb you. Post a "Do Not Disturb" sign if that will help.

You might need to readjust your sleeping habits or your pre-bedtime rituals, but getting enough sleep is the key to having enough energy to successfully get through your workday.