I have recently tried the hmblt Sleep vaporizer pen and it is one of the best oils I have used for sleep. It is on the pricier side, but I am very impressed with the formula that they have produced.
Strains that I personally have tried for sleep would be: Master OG, Granddaddy Purple, Pennywise, Platinum Girl Scout cookies, and Royal Kush. I would say that Master OG made me feel the sleepiest the quickest and that Pennywise is worth noting as it is a CBD strain, adding an extra dimension of relaxation. If any of your sleeplessness is due to anxiety or racing thoughts, a CBD strain will go a long way to help you rest.
Each strain may affect you differently, so you may want to experiment with a few heavy indicas to see which works best with your own chemistry.