The insomnia problem started after a two-week business trip to Los Angeles in January. I was having no problems sleeping through the entire night there. It started the night I returned to Charlotte. Spacing out my anti-depressants by a few days helped resolve the sleep issue, but allowed an episode of depression immediately following a bout of influenza. I'm back on an every-day schedule with the anti-depressants for over a week now, and last night was the first time it's happened again. Fortunately, I'm not letting it bother me too much.
I'm currently walking five miles, two nights a week and that could be having an impact. Exercise means more oxygen in my metabolism, and needing fewer hours of sleep to feel rested each night. Work has also been less demanding, which means I'm not expending an excess of mental and emotional energy every day. To summarize, there's a lot of things that could be causing this, but after experiencing two weeks of the flu and depression, I don't care nearly so much if I'm waking up in the middle of the evening. I just lay there and relax until I fall back asleep, usually within an hour or less.