The Shulchan Aruch (OC 695:2) writes:
One must become drunk on Purim until he doesn't know the difference between "arur Haman" and "baruch Mord'chay".
The Rama (ad loc.) adds:
But some say that he need not get so drunk: rather, he should drink more than his custom and sleep; because he's sleeping, he doesn't know the difference between "arur Haman" and "baruch Mord'chay".
One who does much or little — so long as he directs his heart to heaven!
On the Rama's words "and sleep; because he's sleeping, etc.", Mishna B'rura comments:
That's what's seemly to do.
(All translations are my own.) If one follows this ruling, that sleep is a way — the appropriate way! — to fulfill the "not know the difference" rule, why does one need to drink at all? Is it simply to increase the likelihood he'll fall asleep? If so, can a tired person just lie down without drinking?
EDIT to clarify my question, since I am dissatisfied with the answers thus far given:
There is certainly an idea of drinking wine on Purim to commemorate the miracles that took place through the medium of drinking wine. There is, on the other hand, an idea of la yada, of not knowing the difference between "arur Haman" and "baruch Mord'chay". The Rama explicitly severs the strong connection between these implied by the g'mara's statement "must get drunk on Purim until he doesn't know...": the Rama says that one need not get so drunk as to satisfy "la yada"! (This seems to directly contradict the g'mara.) It is sufficient, he writes, to drink a bit and fall asleep and, by sleeping, satisfy "la yada". There's thus a domino effect, according to the Rama: one drinks sufficiently to sleep, and sleeps sufficiently to "la yada". Yet drunkenness (which is what the g'mara speaks of) needn't lead to "la yada": in fact, one needn't get drunk at all! So the g'mara's requirement "to get drunk until he doesn't know..." is gone. Rama doesn't hold of it. Why, then, does he require drinking to lead to sleep? If the "to get drunk until he doesn't know..." is gone, why isn't it sufficient to simply "doesn't know" (e.g., by sleeping without drinking) (and drink as a separate thing, perhaps after sleeping, to commemorate the miracles)?