Answered by Hans-Ulrich Wittchen
The effects of insomnia – both as a core symptom of depression and as a separate clinical condition – are substantial and have been broadly described (e.g. in the most recent Fifth Edition of the Diagnostic and Statistical Manual of Mental Disorders). There are considerable general (i.e. arousal) and specific (i.e. attention, memory consolidation) effects of insomnia that can impair a broad range of cognitive functions, both in the acute and residual phases of depression.