To define and evaluate sleepiness thoroughly has not yet completely established because of its multidimensional aspects of character. Multiple sleep latency test (MSLT) and maintenance of wakefulness test (MWT) have been the important tools to assess sleepiness in the research and clinical settings, although they have several limitations and pitfalls. The coverage by the National Health Insurance of MSLT, since April 1, 2008, in Japan, so far not MWT, would extend the diagnosis and care of sleep disorders concerning excessive daytime sleepiness including narcolepsy and idiopathic hypersomnia. The relevant and appropriate use of these modalities should be always considered in line with clinical history, medical information and judgment.