Another definition of NIR terminology for you...
Z-score - is a performance criterion. It is calculated by dividing the difference between the NIR predicted result and the reference value by a target value for the standard deviation, for example the RMSEP (but more on that next time!). Z-scores are useful because they convert results to that they can all be compared against the same criteria.
No - it isn't to do with how much sleep you get (which might not have been very much during the recent heatwave here in the UK!)