With the publication of a recent meta-analysis of studies on the relation between disrupted sleep and risk for suicide (Pigeon WR, Pinquart M, Conner K., J Clin Psychiatry. 2012 Sep;73(9):e1160-7. doi: 10.4088/JCP.11r07586. Meta-analysis of sleep disturbance and suicidal thoughts and behaviors.), the fact that disrupted sleep increases risk for suicidal ideation in susceptible individuals can no longer be seriously disputed. Pigeon et al looked at studies on sleep and suicide published since 1966 and covering some 147,753 test subjects. They found that persistent sleep disturbance was significantly associated with an increased relative risk for suicidal ideation, suicide attempt, and suicide and that this increased risk was not due to depression. Insomnia and nightmares appeared to drive the association between sleep disturbance and suicidal risk.
Why does disrupted sleep per se (even without depression in the clinical picture) increase risk for suicide? Both insomnia and nightmares are plausibly due to dysfunction in REM sleep mechanisms, so the primary culprit might be linked to REM sleep malfunction. REM sleep physiological mechanisms are prime candidate sources for the neurocognitive dysfunction of associated with suicidal ideation because 1) REM sleep activation and processing is known to enhance consolidation of negative emotional memories; 2) Nightmares emerge from REM sleep and the intensity of a nightmare is correlated with intensity measures of REM (e.g. REM density); 3) Aberrant REM sleep processes (e.g., reduced latency to REM; changes in REM percentage of total sleep, etc.) are correlated with severity of suicidal ideation linked to depression; 4) REM sleep deprivation can temporarily abolish both depressive affect and suicidal ideation; 5) and awakenings from REM (but not NREM) sleep in healthy people are reliably associated with negative self-appraisals.
I think particularly telling is the repeatedly confirmed fact that both selective REM sleep and total sleep deprivation provides dramatic and immediate (though temporary) relief for some from suicidal ideation. Removing REM removes the cause of the suicidal ideation apparently. Many anti-depressants work via suppression of elements of REM. REM, or at least too much REM as occurs in nightmares and insomnia and in depression, therefore, appears to be a source of dysphoric affect.
Now why should REM be a source of negative emotion? Emotions are more likely to be experienced by the dreamer in REM because REM is associated with heightened limbic system activation and decreased dorsal prefrontal action. Thus, in REM you get a lot of emotion with little critical or rational reflection on that emotion. In people who are vulnerable to impulsive suicide due perhaps to genetic profile or recent stressors or recent depression, a sudden increase in REM percentages or REM indices could signal greater vulnerability to suicidal ideation. Rem suppressant medications or a dose of selective REM deprivation therapy would theoretically relieve the individual of suicidal ideation at least for a few days until REM suppressant medications kick in. But again none of this has been clinically tested so it should not be taken as scientifically proven fact and especially should not be taken as advice for treatment of suicidal ideation. For that you need to see a doctor pronto. These ideas are merely meant to spur innovations in ways to treat the tragedy of suicide.