Funny you should mention a mallet. I have an orange polyurethane mallet full of lead shot a few feet away that I was using to assemble furniture. It shouldn't leave any permanent marks.
I was thinking of health and safety, a rubber mallet would be better than a metal hammer dont you think. Perhaps take a pain killer first. You might not feel so much pain as she gets the swing on the mallet adjusted. Tell her to avoid thin parts of the skull, and perhaps aim around the hair line so the bruises and lumps dont show. Assuming she can aim ok, everything might go well,. If it doesnt work, have more pain killers ready, or tell her to swing harder. If her aim isnt very good ask her to keep her eyes open whilst swinging the hammer, it improves the aim. If she does manage to knock you out, ask her to check your pulse to make sure she hasnt killed you. Maybe you should write her a letter and sign it, stating you had asked her to do this to aid with sleep. It might help in the event the police get the wrong idea and assume she killed you deliberately.
PS Have you tried avoiding sugars or taking any stimulants like caffeine a few hours before you try and sleep. Adjusting your diet may be a safer way of getting to sleep, if you need it. Not everyone needs a lot of sleep, some need only 3 or 4 hours a night. I can manage quite happily on 4 hours when sailing, some people can do less, Single handed sailors maintain a watch via cat napping and suffer no ill health. ie Not everyone needs a full 8 hours sleep.