It has to do with our internal body clock.
Normally, our sleep-wake cycle follows a regular pattern, also known as the Circadian Rhythm and when we sleep "too much" there is a shift in that pattern, the rhythm is interrupted.
Circadian rhythms are the patterns of repeated activity associated with the environmental cycles of day and night. They repeat roughly every 24 hours.
Once our body clocks start "telling the wrong time," we feel it in lethargy, fatigue, and a sleep cycle gone haywire. The clock says one thing and your body says another, very similar to jet lag.
Synchronizing these two clocks (internal and external) come with hitting the "re-set" button every 24 hours.