I agree with Titchou, you shouldn't alter your dosage without consulting your physician.
My daughter has asthma, and one of her many triggers is dust mites. Here is what her allergy/asthma specialist told me:
Buy dust mite proof mattress covers, use one on the box springs too, and one for the pillow. It needs to state it works on dust mites, they are very tiny. Once you zip them on, don't take them off.
Wash all bed linens weekly in hot water, at least 130 degrees, preferably 140.
Vacuum the mattress and pillow covers when you launder the linens.
Vacuum the floors weekly if you have carpeting in your bedroom.
Remove anything near your bed that could harbor mites, such as stuffed animals or clothing.
A room dehumidifier may help as well. They thrive in humid conditions. (I wouldn't purchase one however, unless you know mites are a factor)
If you have drapes/curtains hanging near the bed, clean them as well.
As you already know, the only way to know for certain that dust mites are a trigger for you, is to undergo allergy testing. We were told however, that dust mites are one of the most common triggers of allergic asthma.
I followed all of the above advice (except for the dehumidifier, we found we didn't need it) and my daughter's night time symptoms are nearly non-existent now.
Singular taken at bedtime also has helped many with nighttime problems. You should talk with your Dr.
Hope this helps some,
Shea