I have known times (When I worked in the S.E. of England) during hot dry
summers when we had the irrigation going 24 hours a day, I would change the
lines over at about midnight and my boss would then change them over again
at about 4am. and there was no harm to the plants.
The normal worry is that if the leaves stay wet you can get Mildew and
Botrytis and other fungal infections starting up.
New dahlia catalogue soon on- http://www.abacus-dahlias.co.uk