The Queen was admitted to King Edward VII Hospital on Wednesday night, making it her first hospital stay in eight years.
It comes after the monarch, 95, cancelled a two-day visit to Northern Ireland.
A Buckingham Palace spokesperson said on Thursday night: "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."
It is understood the overnight stay was for practical reasons and the Queen was back at her desk on Thursday afternoon and was undertaking light duties.
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WATCH: The Queen's health over the years after first hospital stay since 2013
The monarch is known for her robust health and her last overnight admittance at the private London clinic was for a nasty bout of gastroenteritis in 2013.
But before this instance, her last hospital stay was in 2003 when she had two knee operations and surgery to remove two small lesions from her face.
The first time the Queen was actually admitted to hospital was in July 1982 when she had a wisdom tooth extracted at the King Edward VII Hospital in central London.
The Queen leaving hospital after her first knee operation in 2003
And in 2018, she was treated as a day patient at the private hospital as she underwent a short procedure to remove cataract from her eye.
The monarch continues to drive and still enjoys riding her horses on her royal estates. Her no fuss approach to illness and injuries was illustrated in 1994, when she broke her left wrist when her horse tripped and they both fell at Sandringham. However, a spokesman at the time said the break was not diagnosed until nearly 24 hours afterwards, as it was first thought to be a bruise.
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The Queen injured her wrist after her horse tripped in 1994
Over the years, the Queen has missed engagements and major events due to flu and back pain but these instances have been few and far between.
She was not present at the Christmas Day service in Sandringham in 2016, when she was suffering from a "heavy cold".
In recent weeks, Her Majesty has undertaken a busy schedule of public outings, including meeting Canadian troops from 1st Regiment Royal Canadian Horse Artillery.
She also launched the Queen's Baton Relay for the Birmingham 2022 Commonwealth Games from the forecourt of Buckingham Palace, and several days later, accompanied by the Princess Royal, attended a Westminster Abbey service of thanksgiving to mark the centenary of the Royal British Legion.
The Queen was pictured using a walking stick at Westminster Abbey on 12 October
It was here she was seen using a walking stick at the Abbey – the first time she has done so at a major event.
She also visited Cardiff, and enjoyed a day at the races at Ascot, presenting the trophy after the Queen Elizabeth II Stakes during the Qipco British Champions Day.
The day before her hospital stay, the Queen enjoyed three engagements – two virtual audiences with the Japanese ambassador and the EU ambassador, and then hosted an evening reception at Windsor Castle to mark the Global Investment Summit.
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