My doctor prescribed 25mg quetiapine for insomnia, is it effective ?

For many, yes: Most often this is prescribed for people diagnosed as bipolar of the type w/ fairly intensely stimulating symptoms. It helps them to be able to lower those so they can relax and sleep. So, this is the effect your doctor hopes will help your insomnia. It gets a 7.3 out of 10 rating by reports from patients for doing this. Only you can decide if it works for you. Best!

It can be.: I usually avoid using atypical antipsychotics for sleep unless other methods fail. How is your sleep hygiene? Retire & rise same time each day. Bedroom should be dark & cool - used only for sex & sleep. Turn off tv. No naps. Exercise regularly but not in late evening. No caffeine for 6 hrs & no alcohol or tobacco w/i 2 hrs of sleep. Dinner should be moderate sized & finished at least 2 to 3 hours