Her admittance is understood not to have been related to coronavirus and she has since returned to Windsor Castle.
The checks came after the Queen cancelled a visit to Northern Ireland on Wednesday.
A Buckingham Palace spokesperson said the Queen went to hospital for “preliminary investigations” but returned to Windsor on Thursday.
The spokesperson 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.”
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.
The Queen’s medical team are understood to have been taking a cautious approach.
The head of state’s trip to central London’s private King Edward VII’s Hospital, which began on Wednesday afternoon, was kept a secret but the Palace issued a statement on Thursday evening after The Sun newspaper broke the news.
It is understood the head of state was due to attend for only a short stay while she was seen by specialists, so the development was not announced by the Palace at the time, and protecting the Queen’s medical privacy was also a consideration.
She was said to be disappointed not to be able to travel to Northern Ireland on Wednesday, and reluctantly heeded the advice of her doctors.