1. A method of detecting and indicating at least one occurrence of a microsleep event experienced by a subject, comprising the steps of: training a neural network to detect said occurrence, said neural network having an input for receiving data, correlated to the occurrence and non-occurrence of microsleep events, from at least one descriptive data source, and an output representing an alertness level result indicating whether a microsleep event has occurred; and, applying a plurality of input feature vectors derived from data received from said descriptive data source to said neural network input, each of said input feature vectors includes data elements corresponding to at least one set of physiological, behavioral or performance data relating to at least one aspect of said subject's alertness state, so as to produce an alertness level result at said output.