Does lack of oxygen cause you to sleep? i've been falling asleep a lot lately.

Depriving : Depriving the brain of oxygen will lead to many effects including increased sleepiness. Why do you think you have a lack of oxygen? This is not a healthy situation and, if it exists, needs to be evaluated by you physician.

Excessive : Excessive sleepiness has a huge number of possibilities and should be evaluated by a doctor.

An : An increase on co2 levels (hypercapnea) has more of a narcotizing effect than relative hypoxemia ( lower oxygen), but either condition is unhealthy. You may very well have obstructive sleep apnea leading to daytime somnolence. You deserve a good cardio-pulmonary examination by your internist, and likely referral for a sleep study (polysomnogram=psg).