Sleep deprivation causes the emotional centers tied to the amygdala to become quite a bit more active. 

“It’s almost as though, without sleep, the brain had reverted back to more primitive patterns of activity, in that it was unable to put emotional experiences into context and produce controlled, appropriate responses,” said Matthew Walker, director of UC Berkeley’s Sleep and Neuroimaging Laboratory and senior author of the study, which is to be published in the journal Current Biology.

This lack of sleep was found to increase amygdala emotional response by over 60%.  It also caused the brain to first connect to the ocus coeruleus, the oldest part of the brain which releases noradrenalin to ward off imminent threats to survival, rather than to the prefrontal cortex. 

This has a certain logic.  A tired animal needs to react quickly to survive.  But, as humans, we need to be able to cognitively assess all situations and this neurological predisposition leads to increased volatility in emotional responses such as anger.  Worse, it increases the likelihood of making inappropriate responses to situations – doing things that we would normally not do in a rested state.

Bottom line – make sure that a coachee is rested before having a heart-to-heart.  And when that is not possible, be prepared to factor in the extra emotional response.

Link:  ScienceDaily