Is it normal to sleep 16 hours sometimes?

No that is abnormal

I do this but I have sleep apnea.

Sounds like you might possibly be clinically depressed.

Exercise, even if it's just walking around outside for an hour or so every day, might help.

You could have a sleeping disorder. Check out Idiopathic Hypersomnia

I barely manage to sleep 4 hours.

doesn't leave anytime for work, are you a democrat?

Studies are saying that too much sleep can definately be as unhealthy as not enough. About 7-8 hours a night is the optimum amount. More than 10-12 hours of sleep a day will cause your muscles, including your heart to lose conditioning faster and weaken, making them more prone to problems, especially if you're getting older. However, if you're getting some good excersise program added to your routine, then longer nights sleep won't be nearly as harmful because you'll be conditioning and strengthening your muscles and heart. If your excersising really hard, long nights sleep are actually good because you're allowing your muscles and heart to recover faster and better from the workouts. But if you are a sedentary couch potato not excersising at all, over 40, and also sleeping 12-16 hours a night, you're asking for trouble.