in·som·ni·a (?n-s?m?n?-?)n. Chronic inability to fall asleep or remain asleep for an adequate length of time.[Latin ?nsomnia, from ?nsomnis, sleepless : in-, not; see in-1 + somnus, sleep; see swep- in Indo-European roots.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.insomnia (?n?s?mn??) nchronic inability to fall asleep or to enjoy uninterrupted sleep. [C18: from Latin, from insomnis sleepless, from somnus sleep] in?somnious adjCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014in?som?ni?a (?n?s?m ni ?) n. difficulty in falling or staying asleep, esp. when chronic. [1685?95;