Evaluated the reverse first night effect (FNE) in 20 insomniacs (mean age 43.5 yrs) showing strong normal FNE and 20 insomniacs (mean age 49.4 yrs) showing strong reverse FNE. Ss were similar in age, sex, and most aspects of reported home sleep. FNEs were assessed using variables that included sleep efficiency, sleep latency, percentage of REM, and REM latency. Measures included the Minnesota Multiphasic Personality Inventory (MMPI), the Self-Rating Depression Scale, the Multiple Affect Adjective Checklist, and the Sensation Seeking Scale. Findings indicate that reverse FNE Ss often appeared to be more suspicious, guarded, and defensive than normal FNE Ss, based on elevated scores on K and Pa in the MMPI. Findings suggest that an insomniac's reaction to sleeping in the laboratory for the 1st time contains important diagnostic information and should not be routinely discarded as an "adaptation." (PsycINFO Database Record (c) 2012 APA, all rights reserved)