If you need a sleeping sprite of one of your characters, it's fairly easy to do with RM's chibi characters. More realistic characters may not work so well with this guide, but it's perfect for RTP. This is only a basic edit; look elsewhere for complex re-spriting. Essentially, what you'll be doing is placing a side-facing head on the front-facing body and redrawing the eye to be closed. This will let your character appear like they're laying with their head off to one side, sleeping (or dead). Nothing real fancy.
Open your sprite in Gimp or Photoshop. Since it's free I'm using Gimp.
Be sure to set the grid size to whatever one of your character's size is. With a 3 by 4 sheet (as for all except XP, I believe), it will be the width of the image divided by 3 and the height of the image divided by 4. For VX Ace RTP(-sized) characters it should be 32*32.
Select the front-facing sprite of your character and go to Layer > Crop to Selection, then Layer > Layer to Image Size.
Erase the head.
Open the original sprite up in a new tab in GIMP, crop everything but one of the side-facing sprites as done in Step 2, and erase the body, leaving only a side-facing head. Select this head and copy with Ctrl+C.
Paste onto previous image.
Right-click on the floating selection in the layers panel and hit "To New Layer"
Move head into proper position. Pixels may need to be redrawn depending on the character, but on such a small scale, minor issues should not be noticeable
Erase eye
Redraw closed eye.
Merge down pasted layer. Zoom out and smile in satisfaction.
Example in-game (two downed with this tutorial and one standing):