1. A pharmaceutical formulation which comprises, in addition to at least one carrier, diluent or adjuvant, at least one neuroleptic compound in an amount effective to exert a neuroleptic effect in a patient requiring such treatment, and melatonin in an amount effective to ameliorate or prevent symptoms of tardive dyskensia developing in the patient. 2. A pharmaceutical formulation according to claim 1 characterized by at least one of the following features: it is adapted for oral, rectal, parenteral or transdermal administration; it is in unit dosage form, each unit dosage comprising an amount of melatonin, which lies within the range of 2.5-20 mg; it is in the form of a controlled release formulation, wherein melatonin is preferably released at a predetermined controlled rate; it comprises also at least one melatonin receptor modifier and/or melatonin profile modifier; said neuroleptic compound is selected from compounds containing at least one of the following ring systems, namely, piperidine, piperazine, morpholine, 5,6,7,8-tetrahydroindole, phenothiazine and thioxanthene. 3. A pharmaceutical formulation according to either claim 1 or claim 2, wherein said neuroleptic compound is selected from among chlorpromazine, triflupromazine, mesoridazine, piperacetazine, thioridazine, acetophenazine, fluphenazine, perphenazine, trifluoperazine, chlorprothixene, thiothixene, haloperidol, loxapine, molindone, clothiapine, clozapine, olanzapine, risperidone and zuclopenthixol acetate, and pharmaceutically acceptable salts thereof. 4. Use of melatonin in the manufacture of a medicament for the prevention or treatment of symptoms of tardive dyskensia in a patient. 5. Use according to claim 4, wherein said medicament takes the form of a pharmaceutical formulation, which includes at least one of the following additional components (a) and (b): (a) at least one carrier, diluent or adjuvant; (b) at least one neuroleptic compound in an amount effective to exert a neuroleptic effect in a patient requiring such treatment. 6. Use according to claim 5, wherein said pharmaceutical formulation is characterized by at least one of the following features : it is adapted for oral, rectal, parenteral or transdermal administration; it is in unit dosage form, each unit dosage comprising an amount of melatonin which lies within the range of 2.5-20 mg; it is in the form of a controlled release formulation, wherein melatonin is preferably released at a predetermined controlled rate; it comprises also at least one melatonin receptor modifier and/or melatonin profile modifier; said neuroleptic compound is selected from compounds containing at least one of the following ring systems, namely, piperidine, piperazine, morpholine, 5,6,7,8-tetrahydroindole, phenothiazine and thioxanthene. 7. Use according to either claim 5 or claim 6, wherein said neuroleptic compound is selected from among chlorpromazine, triflupromazine, mesoridazine, piperacetazine, thioridazine, acetophenazine, fluphenazine, perphenazine, trifluoperazine, chlorprothixene, thiothixene, haloperidol, loxapine, molindone, clothiapine, clozapine, olanzapine, risperidone and zuclopenthixol acetate, and pharmaceutically acceptable salts thereof. 8. Method for the prevention or treatment of symptoms of tardive dyskensia in a patient, which comprises administering melatonin to a patient exhibiting such symptoms or liable otherwise to develop such symptoms, in an amount effective to ameliorate or prevent symptoms of tardive dyskensia developing in the patient. 9. Method according to claim 8, wherein said melatonin is administered as a pharmaceutical formulation, which includes at least one of the following additional components (a) and (b): (a) at least one carrier, diluent or adjuvant; (b) at least one neuroleptic compound in an amount effective to exert a neuroleptic effect in a patient requiring such treatment. 10. Method according to claim 9, wherein said pharmaceutical formulation is characterized by at least one of the following features: it is adapted for oral, rectal, parenteral or transdermal administration; it is in unit dosage form, each unit dosage comprising an amount of melatonin which lies within the range of 2.5-20 mg; it is in the form of a controlled release formulation, wherein melatonin is preferably released at a predetermined controlled rate; it comprises also at least one melatonin receptor modifier and/or melatonin profile modifier; said neuroleptic compound is selected from compounds containing at least one of the following ring systems, namely, piperidine, piperazine, morpholine, 5,6,7,8-tetrahydroindole, phenothiazine and thioxanthene. 11. Method according to either claim 9 or claim 10, wherein said neuroleptic compound is selected from among chlorpromazine, triflupromazine, mesoridazine, piperacetazine, thioridazine, acetophenazine, fluphenazine, perphenazine, trifluoperazine, chlorprothixene, thiothixene, haloperidol, loxapine, molindone, clothiapine, clozapine, olanzapine, risperidone and zuclopenthixol acetate, and pharmaceutically acceptable salts thereof.