The Queen spent Wednesday night in hospital after cancelling a visit to Northern Ireland, Buckingham Palace has said.
A spokesman said she was admitted for "preliminary investigations" - but returned to Windsor Castle at lunchtime on Thursday and "remains in good spirits".
The 95-year-old monarch was treated at the private King Edward VII Hospital in central London, in what was her first overnight hospital stay in eight years.
Sky's royal correspondent, Rhiannon Mills, said Her Majesty's admission was not related to COVID-19 - and she was back at her desk doing some light work by Thursday afternoon.
The Queen had undertaken a string of engagements in recent weeks, but had been given medical advice to rest for a few days.
On Tuesday evening, Her Majesty had attended an event at Windsor Castle, where she was seen alongside Prince Charles, Prince William and Prime Minister Boris Johnson.
During the event, she also shook hands with Bill Gates and US climate envoy John Kerry.
The following day, she had been due to attend a service marking the centenary of the partition of Ireland and the formation of Northern Ireland.
In a statement at the time, Buckingham Palace said the Queen had "reluctantly accepted" the medical advice to rest - adding she was "disappointed" about being unable to make the trip.
Her Majesty's last stay in hospital happened in 2013, when she had suffered a nasty bout of gastroenteritis. She had successful surgery to treat an eye cataract in 2018, and also had a knee operation in 2003.
Next year, the Queen will celebrate 70 years on the throne - and although she continues to carry out official engagements, she has handed more duties to Prince Charles.
Earlier this month, she was seen using a walking stick for support in public for the first time.
The Queen appeared to be in good health during Tuesday's drink reception - and earlier this week, it emerged the monarch had turned down an award that would have named her Oldie of the Year.
According to an aide, Her Majesty believes "you are as old as you feel" - and she felt that she did not meet the "relevant criteria" for the trophy.
Boris Johnson has passed on his best wishes to the Queen as she continues to rest following her overnight stay in hospital, Downing Street has said.
A No 10 spokesman told reporters on Friday: "The prime minister's best wishes have been passed on to Her Majesty."
The spokesman declined to say whether Mr Johnson had had his weekly audience with the monarch.