This is the main reason I prefer a bivvy.Thinking about it, maybe this is why I prefer a bivi to a tent. Not being able to see out does mean your imagination can run wild ...
Didn't stop me pooing myself all night the first time I used it, on the North York Moors, with a mate who's also over 6 foot and much more built than my skinny self. Absolutely nothing happened that was remotely scary (the early morning grouse calls were shocking, but too comical to be scary), but the feeling of relief the following morning at having not been murdered in my sleep was like a second birth. It's probably what got me hooked!
I had quite a strange experience in Northumberland. Bivvying (with the same friend) at a reasonably well-known gathering spot on the summer solstice, there was only one other guy there (this year I counted at least 50). An older guy, probably 50s or 60s, just standing and quietly contemplating, which is what most people do at a summer solstice sunset gathering. We said hello to him but he didn't reply. We sat around and chatted and watched the sun set, but it was kind of eerie him being there. Eventually we went and bivvied about 50 yards away and at some point after dark he must have left. It obviously played on my mind though because when I did finally get to sleep, I had strange and repeated vivid dreams about him returning and all sorts of strange things happening (nothing violent, though, or sexual, for the record).