The British Queen Elizabeth II (95) spent the night on Thursday in the hospital.
The Buckingham Palace announced on Thursday evening.
The reason for the stay was therefore preliminary examinations.
She was back at Windsor Castle by lunchtime and continued to be in good spirits, it said.
She even did light work at her desk.
The PA news agency reported, citing palace circles, that the queen had been examined by specialists in the private King Edward VII hospital and had spent the night in the clinic "for practical reasons".
It was only on Wednesday that the Queen had to cancel a visit to Northern Ireland for health reasons.
A palace spokesman had announced that she had been ordered to rest.
With walking aid at public events
There had already been rumors about the Queen's health last week when she showed up with a walking stick at two appointments.
"It is believed that she first used a walker at an official public event," even reported PA.
The Queen last walked with a stick almost 20 years ago.
But at the time she had recovered from knee surgery.
Elizabeth II has worked stoically through her royal schedule, which was already full again a few weeks after the death of her husband Prince Philip in the spring, in the past few days.
It was not until Tuesday that she received business representatives in Windsor - and looked “fit and happy”, as observers report.