Hypoxia/bradycardia are common symptoms after vaccination of preterm infants. Adults show diurnal variations in vaccination response, due to circadian regulation of the immune system. The investigators plan to investigate whether preterm infants also show differences in hypoxia/bradycardia rate upon morning vs. evening vaccination.

Hypoxia/bradycardia is recorded by pulse oximetry starting 24 hours before until 48 hours after vaccination; parents also kept a sleep-diary. 24 hours after vaccination interleukin-6, interleukin-1β and C-reactive protein get determined. To control vaccination response, pertussis- and haemophilus-titers are determined before vaccination and at 4 months corrected age.

Intervention: First hexavalent vaccination given to very preterm infants either in the evening or in the morning.

Primary outcome: rate of desaturations (SpO2 <80%) and bradycardias (Pulse rate <100/min) in first 24 h following vaccination in evening vs. morning vaccination group Secondary outcomes: cytokine levels (IL 6, IL 1ß, CRP) measured 24 h after vaccination, pertussis- and haemophilus-titers as measured before and after vaccination, i.e. at 4 months corrected age, in evening vs. morning vaccination group

Inclusion Criteria:

• gestational Age: 26+0 to 30+6 Weeks of gestational age

Exclusion Criteria:

• bronchopulmonary dysplasia
• periventricular leukomalacia
• intraventricular hemorrhage >°2
• congenital malformations