The woman was taken into custody on Wednesday after investigators found a syringe containing breast milk and traces of morphine in her locker at Ulm University Hospital in southern Germany.
The babies, who were between one day and five weeks old and staying in the same hospital room, all suddenly developed breathing problems "at almost the same time" in the early hours of December 20, Ulm police chief Bernhard Weber said.
"Only because of immediate action taken by the staff could the five lives be saved," he told a press conference.
Doctors do not expect the infants to suffer any lasting harm.
The young nurse taken into custody has yet to be formally charged but faces five counts of attempted manslaughter and grievous bodily harm, Ulm prosecutor Christof Lehr told reporters.
She denies poisoning the babies, he added.
Hospital staff initially suspected the infants had caught an infection, "but this could be ruled out after urine tests", police chief Weber said.
The tests did however show traces of morphine -- although two of the babies had not been prescribed the heavy painkiller as part of their care.
The hospital notified the police on January 17.
A search of the lockers of employees on duty around the time of the incident turned up the syringe "filled with breast milk", said Lehr.
Testing "confirmed the terrible suspicion that the syringe contained morphine", he said.
Prosecutors believe the woman acted with premeditation and "accepted that the babies could die" as a result of her actions, Lehr said.
Her psychological state is currently being evaluated.
- 'Sympathise with parents' -
Morphine is a powerful drug administered to treat severe pain. An overdose can lead to life-threatening respiratory failure.
Ulm University Hospital said "rapid medial intervention" meant the babies' conditions "were stable again within 48 hours and they could breathe on their own again".
"We very much regret that this incident could have happened and we sincerely apologise to the parents and children," a hospital statement said.
"We can all very much sympathise with the parents' concern for the health of their children."
More than 400 people work in the hospital's paediatric and adolescent care units.
The incident revived memories of Niels Hoegel, a German nurse jailed for life last year for the murder of 85 patients.
Hoegel, believed to be Germany's most prolific serial killer, murdered patients with lethal injections between 2000 and 2005, before he was caught in the act.
He admitted to the injections that caused heart failure or circulatory collapse so he could then try to revive patients to impress colleagues.
"We very much regret that this incident could have happened and we sincerely apologise to the parents and children," a hospital statement said.
"We can all very much sympathise with the parents' concern for the health of their children."
More than 400 people work in the hospital's paediatric and adolescent care units.
The incident revived memories of Niels Hoegel, a German nurse jailed for life last year for the murder of 85 patients.
Hoegel, believed to be Germany's most prolific serial killer, murdered patients with lethal injections between 2000 and 2005, before he was caught in the act.
He admitted to the injections that caused heart failure or circulatory collapse so he could then try to revive patients to impress colleagues.
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