In my case an iodine deficiency interfered with my sleep. I now take a potassium iodide supplement.
What wakes you after (less than?) four hours? I can't sleep longer than that because I have to urinate that often at night. This is common at my age. I usually go back to sleep when I return to bed.
Do you have a similar problem? Explain - more detail, will help readers decide if they have any helpful experience.