No, you can't stockpile sleep. In other words, there's a limit to how much you can sleep, and your body won't allow you to sleep more or carry sleep forward. Staying in bed and trying to sleep longer once you're fully-rested won't provide you with additional benefits. Additionally, most of your slow-wave sleep happens during the first part of sleep. That's why the first half of your night is so important, and why sleeping longer (beyond what your body needs) has diminishing returns.