Long-term efficacy and safety of zolpidem extended-release 12.5 mg, administered 3 to 7 nights per week for 24 weeks, in patients with chronic primary insomnia: a 6-month, randomized, double-blind, placebo-controlled, parallel-group, multicenter study - Krystal,A.D.; Erman,M.; Zammit,G.K.; Soubrane,C.; Roth,T. (Sleep, 2008)