Two different series of urea diamides have been synthesised by keeping X=O/S to form urea/thiourea derivatives in 2 and 4 substitutions in the phenyl rings by carboxylic acid and carboxamide to form diamides for screening of CNS depression and sleeping time synergistic activity on mice with reference standard drugs benzodiazepine and barbiturate. It has been found that all the test compounds have sleep inducing property due to the presence of open chain urea/thiourea linkage and the urea/thiourea diamide derivatives were found longer duration of sleep inducing property due to the presence of two amide linkages and out of which the thiourea derivative and thiourea diamide derivatives showed lesser duration than urea and urea diamide linkage. Sleeping time potentiation effect was studied for the test compounds by using pentobarbitone as barbiturates and diazepam as benzodiazepines on male albino mice. The 2-substituted derivatives were found less active than 4-substituted derivatives due to the steric hindrance and ortho effect. All the observations were noted for four groups of mice and the bioassay result was tabulated after statistical parameters for significance of pharmacological screening: Student’s-t-test and P-value. The CNS depression occurs due to the action on GABA receptor having free NH2 and COOH groups: H2N-CH2-CH2-CH2-CH2- COOH (ã-amino butyric acid). The synthesized compounds which have free ?C(O/S)-NH2 (urea/thiourea) groups and ?COOH groups shows the CNS depression effect and the maximum activity has been shown by the compound-6 in which amido as well as urea linkages are in para position to each other which blocks the GABA receptor and results CNS depression. The same attachment shows lesser action in case of ortho substitution due to ortho effect and steric hindrance. All the test results were found significant to a high extent for the structure activity relationship studies of the synthesized molecules and the maximum activity has been shown by the COMPOUND-6 (4-Amido phenyl urea) having two amide groups.