depression
"My husband has bi-polar disorder and suffers from chronic back / sciatic work-related pain. He'd only been (restlessly) sleeping 2-3 hours a night for past 10 years. This had increased his mood swings and cyclothymia to unmanageable proportions!!
UK doctor prescribed 10mg amitriptyline, to be taken at night 2 hours before bed. After two weeks this was increased to 20mg per night.
The effect has been nothing short of miraculous.
He sleeps really well for 7-8 hours a night, wakes refreshed & cheerful.
It's helped him cope differently with depression - staying calm & getting CBT therapy. (Something he'd never have volunteered for in his 'normal' bi-polar anger / depression cycling).
Has also noticed lessening of pain. 
It's saved our marriage."