There is no specific contra-indication for taking an antihistamine (to which class
Diphenhydramine belongs) and
Champix. Therefore, you may take it once in a while.
Some tips for sleep problems are:
Keep fixed waking time each day as far as possible.
Don't sleep for the first six hours after waking up.
Avoid day time naps. If you can't avoid, keep day time naps to about half an hour.
Exercise regularly.
Avoid TV / computer screen or mobile screen for an hour before your sleep time.
Go to bed only when sleepy.
Avoid bright light and noisy room for sleep.
Don't take tea/coffee in evening/night.
Don't lay in bed, if you can't sleep. Sit up and listen to music or read in soft light. And lie down again when sleepy.
Hope this answers your question.