Every morning in מודה אני we say שהחזרת בי נשמתי - You returned my neshama. You know how you make a deposit in the bank? The more money you put in the bank, the more you will find when you withdraw the money. It is fun depositing, let's say, 50 thousand dollars [not that I've tried that before but I use my imagination] because you know that next time you take money out of the bank you will have 50 thousand ready and waiting. Baruch Hu U'varuch Shemo!
When you go to sleep at night you "deposit" your neshama with Hashem. In the morning you receive the SAME neshama you deposited. The more polished and refined the neshama, the more satisfying the "withdrawal" will be. This is why many have a custom to say vidui [confession of sins] before going to sleep and of course say krias shema al ha-mittah with great kavana. We want to give Hashem the purest neshama possible so that when we awaken we will be receiving a pure, elevated neshama.
מודה אני לפניך מלך חי וקיים שהחזרת בי נשמתי
ע' בסידור עולת ראיה