Sleep disorders nosology is rather complex, but it is important to be aware of major categories. Understanding the classification is useful in navigating this complex topic. Below are the major categories with common examples.
- primary insomnia
- insomnia due to mental condition, medical condition or substances
- behavioral insomnia of childhood
- Sleep-related Breathing Disorders
- central sleep apnea
- obstructive sleep apnea
- sleep-related hypoventilation/hypoxemic syndrome
- Hypersomnia of Central Origin
- Kleine–Levin syndrome
- menstrual-related hypersomnia
- Circadian Rhythm Sleep Disorders
- jet lag, shift work disorders
- Disorders of arousal from NREM: sleepwalking, sleep terrors
- Disorders of REM sleep: REM sleep behavior disorder, recurrent isolated sleep paralysis, nightmares
- other: enuresis, sleep-related eating disorder
- Sleep-related Movement Disoders
- restless leg syndrome
- periodic limb movement disorder
Sleep and Depression
Depression causes a number of sleep disturbances: increased sleep latency, increased nocturnal and early-morning awakenings, shortened latency to REM sleep, increased REM density, decreased slow-wave sleep in first NREM cycle.
REM sleep deprivation alleviates depression; also most antidepressants suppress REM sleep (exceptions: bupropion, nefazodone (Serzone)) (K&S, p.2154)
These are believed to have activating effect and offer require co-prescribed sleeping aid :
- imipramine, desipramine (TCAs),
- reboxetine (not effective for MDD in adults according to meta-analysis) 
These are believed to promote sleep through resynchronization of the circadian rhythm (not through sedating action)
- doxepin, trimipramine (TCAs),
- agomelatine (Valdoxan, Melitor) (great for improving sleep AND severe depression!) 
fluvoxamine (Luvox) and paroxetine (Paxil) are generally considered sedating.
Sleep and Alcohol
Alcohol is a sedative and small amounts will help with falling asleep. Having more than 1-2 drinks at bedtime is likely to results in the following sleep disturbances:
- supressed REM sleep
- supressed stage 4 sleep
- increased fragmentation of sleep (as a result of the two factors above)
- vivid dreams at the end of the night (as blood alcohol levels fall)
Heavy drinkers may experience continued disruption of sleep even after several months of abstinence.
 Kaplan and Saddock Psychiatry
 Wichniak A., Sleep and Antidepressant Treatment. Curr Pharm Des. 2012
 Cipriani A., Comparative efficacy and acceptability of 12 new-generation antidepressants: a multiple-treatments meta-analysis, Lancet, 2009
 Montgomery, SA., Severe depression and antidepressants: focus on a pooled analysis of placebo-controlled studies on agomelatine.". Int Clin Psychopharmacol. 22 (5): 283–91.