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Abnormal sleep can be insufficient, which constitutes insomnia; it can be excessive or inappropriate, which is hypersomnia, it can be misplaced with respect to the normal sleep-wakefulness rhythm, or it can be accompanied by various phenomena which do not belong to sleep itself (i.e. parasomnia). This is the format of most traditional classifications of sleep disorders. A modern classification has been introduced in 1990 and contains the description of 88 different sleep disturbances. They are classified in the main categories of dyssomnia, parasomnia and sleep disorders associated with psychiatric, neurological or somatic conditions. This classification is an excellent tool for the practice of sleep medicine and for the study of sleep disorders.

During one year, free and total tryptophan, tyrosine and non-esterified fatty acids were determined in the blood of eight healthy volunteers. No clear seasonal rhythm appeared for any of these parameters. But over this period, the following significant correlations were found: total tryptophan-free tryptophan; total tryptophan-tyrosine; free tryptophan-tyrosine. For the individuals, the quotient tyrosine-tryptophan showed to be constant during this year.