[Legolas]

As a footnote, Sominex in the USA contains Diphenhydramine rather than Promethazine, whilst the USA version of Benadryl also contains Diphenhydramine rather than Acrivastine or Cetirizine as in the two UK versions.
They're all working in more or less the same way, though some will be in your body longer than others. Also, exactly which side effects are more likely to manifest may vary.
Acrivastine and Cetirizine are a lot less sedative overall, though both of them might still do the job. Try not to drive a car on days after using any of these.

If you do want to use the Diphenhydramine to get some sleep, you might see if a lower dosage is still effective enough to sedate your brain without triggering nightmares in the same organ.