Does anybody know what is the cause of sleep spindles/k-complexes, I have done research and know they occur during theta sleep but havn’t come across the cause.
Here you go Lucidity_Master:
[size=142]Sleep spindle
From Wikipedia, the free encyclopedia
A Sleep spindle is burst of brain activity visible on an EEG that occurs during stage 2 sleep. It consists of 12-16 Hz waves that occur for 0.5 to 1.5 seconds.
Sleep spindles are one of several measures employed by the brain to keep a sleeper in an unconscious state. This pattern is produced during sleep to inhibit the processing of unnecessary information, which would especially interrupt stage 2 sleep.
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That clears it up. 
Thank you