My sim would sleep but suddenly she would wake up for no reason.

If you have removed all the previously mentioned possible noises and checked you have a door not an archway to the bedroom. You could try zooming in until you are basically hovering over your sim, make sure your audio settings are set to high for this testing and that your speakers or headphones are on a reasonably high level (don't hurt your ears!), and hit play. Don't fast forward for this. This allows you to listen to what your sim can hear in the game. When they woke up, did you hear anything?  

Also, I would start with looking at the bed's comfort rating. if it's high, then continue troubleshooting:

Troubleshooting Suggestions:

• When they wake up, hit pause and expand all their moodlets. Do you see anything with a negative moodlet? 
• As they sleep, as they wake up, and as they do whatever they do right after waking up - What are the images in their thought bubbles? It would be reasonable to write those down and post them in the forum if you need help brainstorming possible interpretations of the images from thought bubbles.