What accounts for unrefreshing sleep?
Often we think of reductions in total sleep time and fragmented sleep as the major contributors to feeling unrefreshed the next day. While this is true (and has been shown extensively in controlled studies) it is possible to have normal total sleep time but for alterations to the composition and distribution of certain stages of sleep to affect how we feel and function. For example, a recent study found that 'knocking-out' deep- or slow-wave sleep (by using sound to shift the brain into lighter phases of sleep) impairs the ability to form new memories the next day and leads to increased lapses during attention tasks. It has also been shown that micro-changes in the scalp-recorded Electroencephalography (EEG) (the main measure of objective sleep) can be associated with poor sleep quality, without there necessarily being any clear reductions in total sleep time
So in short, the quality as well as quantity of our sleep is what matters – and it's why a reliable schedule is a critical part of being a healthy sleeper.
See this article for more information on getting a good night's sleep