I Have What I Believe Is Stress Hair Loss In

I have what I believe is stress hair loss in the front and I recently bought Minoxidil 5% spray to try to help this. I lost some when my husband died and more recently I moved, had leg surgery and my mother passed away in just a few months and I began thinning again. The directions were to apply the spray for 2 to 4 hrs and then wash it out 2 x a day. I do that in the morning but leave it in at night till morning. My hairdresser said I should check because leaving it in at night might be harmful. Is it? And am I just wasting time with this stuff?