How are melatonin levels measured in humans?
On the analytics of melatonin I have here a german language text by the RKI with references. Translated quote:


  Assay of melatonin is done by serum or sputum using RIA or ELISA.
  Alternatively, to infer the nightly amount of melatonin secretion it
  is possible to look for the melatonin catabolic metabolite 6-hydroxy
  melatonin sulfate (6-OHMS) in urine.[1,2] ... standardized
  conditions of sample taking are a must to get good results.[3,4]


Ref.:


P. Levallois, M. Dumont et al: Effects of electric and magnetic fields from high-power lines on female urinary excretion of 6-sulfatoxymelatonin. In: American journal of epidemiology. 154, 7, October 2001, 601–609. PMID 11581093
D. H. Pfluger, C. E. Minder: Effects of exposure to 16.7 Hz magnetic fields on urinary 6-hydroxymelatonin sulfate excretion of Swiss railway workers. In: Journal of pineal research. 21, 2, September 1996, 91–100. PMID 8912234
E. Gilad, N. Zisapel: High-affinity binding of melatonin to hemoglobin. In: Biochemical and molecular medicine. 56, 2, December 1995, 115–120. PMID 8825074 
D. K. Lahiri, D. Davis et al: Factors that influence radioimmunoassay of human plasma melatonin: a modified column procedure to eliminate interference. In: Biochemical medicine and metabolic biology. 49, 1, February 1993, 36–50. PMID 8439449