Sleeping Pill Overdose
Some hypnotics — the active component of sleeping medications — act as central nervous system (CNS) depressants, which in turn depress breathing or respiration, especially when mixed with other drugs, such as alcohol. 

If your breathing slows sufficiently and is arrested, as can be caused by an overdose of CNS depressants, your vital organs will be starved for oxygen, which in turn can lead to brain damage and — ultimately — death.