The Queen spent Wednesday night in hospital for preliminary medical checks and is now back at Windsor Castle, Buckingham Palace has said.
The 95-year-old monarch returned from the private hospital in central London at lunchtime on Thursday and is "in good spirits", the palace added.
She was given medical advice to rest for a few days after a busy schedule of public engagements.
In a statement on Thursday night, Buckingham Palace said: "Following medical advice to rest for a few days, the Queen attended hospital on Wednesday afternoon for some preliminary investigations, returning to Windsor Castle at lunchtime today, and remains in good spirits."
The Queen travelled by car to the King Edward VII's Hospital in Marylebone, about 19 miles (32km) from Windsor, where she was seen by specialists. Her admittance is understood not to be related to coronavirus.
The overnight stay was said to be for practical reasons and the Queen was back at her desk undertaking light duties on Thursday afternoon.
It is the first time the Queen has stayed in hospital since 2013, when she suffered symptoms of gastroenteritis.
Prime Minister Boris Johnson's official spokesman said: "The prime minister's best wishes have been passed on to Her Majesty."