Inhaling eucalyptus oil in moderation is healthy and safe although if you do inhale too much eucalyptus oil then it could lead to seizures, brain damage and possibly death but you'd have to inhale a whole lot of the eucalyptus oil for it to be a problem.
Inhaling too much eucalyptus oil could also get you high and make you hallucinate as well and you'll usually either start feeling high or feeling sick as a result of inhaling too much eucalyptus oil.
Just inhale the eucalyptus oil in moderation and you should be okay but I would avoid inhaling eucalyptus oil more than once or twice per day just to be safe.
I inhale eucalyptus oil sometimes at night when I cannot go to sleep and it helps me fall asleep fast and helps me stay asleep.