This person is in the running for a Well Done, Sherlock! award. Dr Sara Mednick, who has a Phd in Psychology from Harvard and is a researcher and faculty member at the University of California, has figured out that taking a nap in the afternoon can be quite refreshing. She's even written a book about it, available from our tax-dodging friends at Amazon for a mere £8.99.
Her researches have revealed, amongst other things no doubt, that there is an appropriate time during the day, which is related to the time you woke up, to take 'The Ultimate Nap'. This is a perfectly balanced state in which REM and slow-wave sleep are equally proportioned. There's a nifty little Flash animation on her website that you can use to work out when this will be in your particular case. This assumes, of course, that you have had an unbroken night's sleep and Dr Mednick does not explain on her website what to do if this is not the case. Should one take a nap according to the first time you woke up or the latest? Or a nap for each waking up? I guess we'll have to buy the book to find out.