I have been a Reiki Healer/teacher for 16 years, and over the past 2 years have done a lot of high energy work, which has also meant a need for a lot of protection, which should be more than adequate to cover all eventualities

11 months ago I moved to the Isle of Wight, where the energy is very different from the mainland.  Its absolutely wonderful but has taken quite a while to adjust.  The Island has many ghosts, some of which I have become aquainted with, both good and not so good energies.

However, over the last couple of months, I have been attacked in the middle of the night, by what I can only explain as black energy.  It is not a ghost or spirit and that is as far as I can explain it.  The first attack I dismissed with Reiki symbols in my sleep - no problem.  The second was much stronger, but I still was able to protect myself with the Reiki, but it was a bit scarey and it did wake me up.  I placed a piece of black tourmaline under my pillow on  my left and have a quite large citrine/quartz twin pointed crystal the other side, which invariably I hold when I sleep.

This appeared to keep the entity at bay, but it has reappeared twice in the last couple of weeks, nowhere near so strong and is easily dismissed.  

I'm not frightened, can handle it OK, but does anyone have any idea as to what is happening - and is there anything else I can do.

The entity is not resident in the house which has a lovely energy and just seems to appear at around 2.00am

did you open up a portel of some kind while doing something reiki related?

Have you made anyone mad  or forgot to do a complete cleansing and closing in your work .

I think you might be experiencing what I'll call 'etherial rundown'.  You mentioned that  you've been doing 'energy' work for several years, could it be time for a well deserved vacation? BY THAT I mean taking the time to rejuvenate yourself and build your etherial 'muscle'.  I've had  a similar experience, but the energy did not enter my home, it  hovered outside, as if it were stalking me.  I too did not think of it as an entity, but as a 'dark' energy.
At first I was concerned, but then I came to my senses.  The reason it was lurking outside was because I don't allow anyone or anything in my home that makes me feel uncomfortable.  By keeping your etherial body strong and setting bounderies you should have no problems with either entites or 'black' energies.  I make it known that  entities are guests whose welcome can be revoked at any time I wish, no matter where we meet.  As far as what it is, I can only guess.  You say it comes around 2:am.  Could it be someone with a serious grudge (ex lover?) who astral travels (whether or not they know it) or possibly 'drinks and thinks' (bars usually close around 2:am) It could be that this person dosn't even know that they're doing this. Like you, I too would be curious (you could try using a pendulum for the answer) but your first concern should be to stop it.  If you do figure it out, I'd love to here about it. Blessed be.