Breast milk will be given to the newborns in the intervention group for 3 days in line with the "feeding model with a chronobiological approach", simultaneously with the mother's supply.

Breast milk will be given to newborns in the control group according to the clinical routine and infants will be followed up for 3 days.

Breast milk will be given to the newborns in the intervention group for 3 days in line with the "feeding model with a chronobiologic approach" simultaneously with the provision from the mother, and the newborn will be ensured to receive breast milk in accordance with the day cycle. Within the scope of the study, firstly, mothers and nurses in the NICU will be informed about the study. Mothers will be asked to put the milk mothers express between 06:00-17:59 in the "daytime breast milk storage bag" and the milk mothers express between 18:00-05:59 in the "nighttime breast milk storage bag" and bring them to the unit. The brought breast milk will be prepared by the nurses by matching "day and night" milk according to the dates, with the first milk brought in priority, and the newborns will be fed by using the "time of day factor" (cycle so that the milk collected during the day will be given during the day and the milk expressed at night will be given at night).

The form prepared by the researcher in line with the literature (Galland et al., 2012; Huang et al., 2021; Caba-Flores et al., 2022) consists of questions including the obstetric characteristics of the infant such as gender, gestational-postnatal age, type of delivery, APGAR scores, birth weight, height, head circumference, and the current clinical status of the infant such as diet, comorbidity, and total number of hospitalization days. The questionnaire form was sent to 5 academicians specialized in Pediatric Nursing, their expert opinions were obtained, and the questionnaire form was finalized by making the necessary changes in line with their suggestions. Information about the newborns in the control and experimental groups was collected by the researcher through the "Newborn Introductory Information Form" before the study, hospital records of the newborn and the primary nurse through face-to-face interview method will be collected in about 5 minutes.

"Temperature (°C), Pulse Rate (min), Blood Pressure (mmHg), Respiratory Rate (min), O2 Saturation (mmHg) will be recorded for 3 days during the day-night cycle of the infants followed in both groups. Physiologic parameters will be measured 1 hour after the 2nd daytime feeding and 1 hour after the 2nd nighttime feeding (i.e. after the baby's 6th feeding). The amount of breast milk received by the infant (ml)" will be monitored for three days separately in the daytime (06:00-17:59) and nighttime (18:00-05:59) cycle. In addition, the infants will be followed up in terms of "Body Weight (g), Height (cm), Head circumference (cm), Persentile" values until the infants are discharged from the hospital and all these parameters will be recorded through the "Newborn Growth and Physiological Parameters Follow-up Chart" created by the researcher.

At the end of 3 days, the records of the duration of the babies' being in "Wakefulness, REM, N-REM / EVRE I, N-REM / EVRE II, N-REM / EVRE III, N-REM / EVRE III, N-REM / EVRE IV" measured by Bispectral Index (BIS) in 5-minute periods within a 24-hour time cycle will be taken from the BIS monitor records and entered into the "Newborn Circadian Rhythm Follow-up Chart" created by the researcher.

"Temperature (°C), Pulse Rate (min), Blood Pressure (mmHg), Respiratory Rate (min), O2 Saturation (mmHg) will be recorded for 3 days during the day-night cycle of the infants followed in both groups. Physiologic parameters will be measured 1 hour after the 2nd daytime feeding and 1 hour after the 2nd nighttime feeding (i.e. after the baby's 6th feeding). The amount of breast milk received by the infant (ml)" will be monitored for three days separately in the daytime (06:00-17:59) and nighttime (18:00-05:59) cycle. In addition, the infants will be followed up in terms of "Body Weight (g), Height (cm), Head circumference (cm), Persentile" values until the infants are discharged from the hospital and all these parameters will be recorded through the "Newborn Growth and Physiological Parameters Follow-up Chart" created by the researcher.