(S (S (NP (BASENP (DT An) (NN analysis)) (PP (IN of) (BASENP (NN melatonin) (NNS studies)))) (VP (VBZ has) (VP (VBN upheld) (NP (NP (BASENP (DT the) (JJ controversial) (NN supplement) (POS 's)) (NN effectiveness)) (PP (IN as) (BASENP (DT a) (NN sleep) (NN aid))))))) (PERIOD .))