My oxygen starts falling just start to doze off, not asleep. It drops to 89. not sleep apnea becahuse im in light sleep. What does this mean?

Hypoventilation . If you are overweight it can be related to Obesity Hypoventilation syndrome and this goes hand in hand when you have Sleep Apnea. Many Pulmonary diseases can do that too if you have any. Have a clinician see you for definitive answer.