Thinking about the times of day we say the Shema, the Baal HaTanya points out that the bowing down in the Amidah is the essence of actually receiving the yolk of heaven. So what happens when we say the Shema, essentially voicing our desire to accept the yolk of heaven, but then we don't follow through with the Amidah? Namely, when we say Kriath Shema before going to sleep.
If we pay attention to what the Baal HaTanya explained the answer is apparent: The bowing is where the acceptance happens. Bowing is a form of bitul, of belittling and nullifying ourselves before HaShem. What could be more nullifying and belittling than sleep? It is as if we cease to exist altogether.
In fact, one of my favorite midrashim comes to our rescue here. Midrash Rabba on bereishith explains that when HaShem created Adam, the angels couldn't tell Adam appart from HaShem. (Presumably since he was made in the image of HaShem) In the end, HaShem put Adam to sleep which made it clear to the angels that Adam wasn't HaShem, since HaShem never sleeps. Adam was nullified before HaShem via the vehicle of sleep.
So now we can look at the act of going to sleep in an entirely different fashion. Rather than going to sleep because we are exhausted, we can see going to sleep as an act of nullifying ourselves before HaShem. It is a testament to the fact that HaShem runs the world and when we sleep the world continues to exist and function. This is why we say Shema before sleep, to prepare ourselves for the awesome revellation of HaShem that happens in sleep even more than when we are awake and bowing in the Amidah.
If we recall a long time ago, we mentioned Rebbe Nachman's teaching (see: sleep: the answer to every question) brought down in the beginning of Likkutei Halachoth that through sleep we are nullifying ourselves before HaShem and that is why we awaken with new life, refreshed. In that Torah we learn that sleep is one of the biggest tikkunim, a powerful way to fix what we have damaged in the past. [to delve into this a little deeper see: sleep as daat instead of a disconnect]
Perhaps we can even touch the level of dying for Kiddush HaShem (in the name of HaShem) when we go to sleep as an act of bitul before HaShem. After all, chazal teach us that sleep is one sixtieth of death.