You are of course refering to the final words of this powerful poem, where the alter-ego of the speaker of the poem says to the speaker that they are able to "sleep" together now after identifying himself as the enemy that the speaker killed just the day before:

I am the enemy you killed, my friend.
I knew you in this dark: for so you frowned
Yesterday through me as you jabbed and killed.
I parried; but my hands were loath and cold.
Let us sleep now...

The sleep therefore represents the way that the speaker of this poem realises that he, too, is now dead and that this sleep that his enemy/friend invites to share with him is the sleep of death. It is important however to recognise how this ending of a common death for the speaker and his enemy/friend actually reinforces the central theme of the poem. This poem is above all a moving remembrance of all soldiers of many different nationalities who lost their lives in a war that very few understood. The ending of the poem represents the conclusion of a journey for the speaker and his alter-ego, as they are now able to "sleep" as comrades together, even though the uniforms that they had on indicated that they were enemies. These superficial differences have been overcome by the powerful sense of brotherhood that is proclaimed in the poem. In death, therefore, these two soldiers are able to rise above the petty divisions that set them one against the other in life.