Why do I bite my tongue when im a sleep and the pain wakes me up?

Tongue too large. Your tongue could be a bit too large for your mouth or your lower jaw might be small. Perhaps you grind your teeth in sleep (bruxism). Do you snore? If your tongue is too large for your mouth, that increases the chances that you snore and even have sleep apnea (stopping breathing in sleep), which can be a dangerous condition and requires treatment by a sleep medicine specialist.
Possible bruxism. This could be due to a habitual bruxism (clenching or grinding). A dental appliance (such as a hawley bite plane) that separates the back teeth by preventing them from meeting can protect your tongue from injury and allow a better, uninterrupted night's sleep.
Grinding. It is hard to say. You may have a sleep disturbance such as brushing, grinding, snoring, sleep apnea, or others. First see a dentist for an evaluation, and secondly you may need to see a sleep specialist.