SUMMARY Samples of saliva (4 in 24 hours), collected at monthly intervals for the first 6 months of life in 8 term infants by their mothers, were analysed for cortisol by radioimmunoassay. Values in the first month were more variable, daily mean values were greater, and amplitudes of variation were greater than in subsequent months. The circadian rhythm appeared by the third month.The age at which the 'adult' circadian rhythm of cortisol secretion emerges is difficult to ascertain for obvious ethical reasons and the few data available all conflicting. Franks' measured plasma 1 7-hydroxycorticosteroids at 8 am and 8 pm in children and concluded that the 'adult' pattern emerged between 1 and 3 years. A study, however, based on urine collections every 3 hours throughout the day in male infants suggested a diurnal rhythm of excretion of 17-hydroxycorticosteroids by day 26 of life.2 The most complete study, by Zurbrugg,3 who took blood samples every 4 hours from 6 neonates (3-9 days old), from S infants (1-6 months old), and from 11 children (1j-1 1 J years old), concluded that 12 hourly cycles of plasma cortisol occurred in the neonatal period and a 24 hour cycle in the second year of life. The mean daily plasma cortisol concentration and mean amplitudes of variation were comparable in all age groups.Mixed salivary cortisol values closely reflect plasma free cortisol values4 and a circadian rhythm has been identified in adults5 6 and children.7 To determine when the circadian rhythm appeared an ethically acceptable longitudinal study was carried out in 8 normal infants in their homes, using small salivary samples taken by mothers for cortisol estimation.
Patients and methodsEight term neonates (4 boys and 4 girls), whose mothers agreed to participate in the study, were selected. Four deliveries were unassisted, 1 was forceps assisted, and 3 were by caesarian section.All babies were of normal birthweight (2.95-3.5 kg) except 1 who was light for gestational age (2 3 kg at 41 weeks). Nonie ofthe babies had postnatal problems and all were well during the study. Five were breast fed. Mothers were asked at what age the night feed was omitted and babies slept throughout the night.Saliva collection. Mothers were requested to provide 4 samples in a 24 hour period during weeks 1,4,8, 12, 16,20, and 24 of life. Several collections requested for week 24 were made between weeks 24 and 30. The first sample was requested at a convenient time between 6 am and 8 am, the second between 11 am and 1 pm, the third between 3 pm and 6 pm, and the final sample between 10 pm and 12 pm. Mothers were instructed to place a small citric acid crystal on the infant's tongue and aspirate 1-3 ml of saliva with a disposable mucus extractor over the next few minutes.8 Saliva was transferred to small plastic capped bottles (with the times recorded) that were sent to the laboratory on the following day and stored at -20°C until analysed.The project was approved by the local ethical committee. On consultation during and after the study mothers did not ...
