How safe is it to use essential oils and to sleep with a diffuser on? could i be exposed to too much?

Generally safe: generally safe. I normally use a few drops in a dish and let it sit in a room instead of using a diffuser. using a diffuser should be ok. if you are concerned, you can dilute the essential oil with olive oil. also google safety of essential oil. lots of good info out there

Very safe, possible: Essential oils tend to be very safe when inhaled by a diffuser. Problems are more likely if applied to skin (due to irritation) or if taken internally. But too much of anything can be hazardous and too inhaling too much essential oils could irritate your nose or lungs or cause nausea, headaches, dizziness or fatigue. See http://www.floracopeia.com/education/how-to-use-essential-oils-effectively/