It's natural to point the toes down slightly - as in the angle you see if you're hanging your feet off the bed. But pointing them down more than that will tighten your Achilles tendon, and will leave you vulnerable to tearing it. Believe me, that is a Bad Thing(tm). Even a small tear is very painful! A minor tear will heal in, oh, a couple of months, but in extreme cases it requires surgery. I definitely do not recommend doing anything to risk it!