The world's oldest and longest-reigning monarch spent a night in the hospital on Wednesday but returned to Windsor Castle the next day. Officials said she was in good spirits and back to work. The queen had been advised by medical staff to rest, but her illness was not related to COVID-19. She attended hospital for preliminary investigations on Wednesday and returned to Windsor Castle on Thursday. The palace did not provide details on the reason for the medical attention, and some royal correspondents hoped for a more complete explanation. Prime Minister Boris Johnson sent his best wishes and said the queen was back working at her desk. Johnson informed reporters that Queen Elizabeth was back working at her desk in Windsor, according to a royal source. The source explained that the queen had stayed at the King Edward VII hospital in London for practical reasons and that her medical team had been cautious. The source mentioned that she is now resting and performing light official duties. Despite being queen of multiple realms, including Australia, Canada, and New Zealand, Elizabeth returned to work on Thursday afternoon and was handling light duties, the source added. Elizabeth, who became queen during a time when Britain was losing its imperial power, has represented stability to generations of British people and has helped maintain the popularity of the monarchy despite significant political, social, and cultural changes that could have made it seem outdated. Her quiet and diligent commitment to her duties, even in her old age, has earned her respect both in Britain and abroad, even from those who wish to abolish the monarchy. Elizabeth hosted a drinks reception at Windsor on Tuesday night for billionaire business leaders, including Bill Gates, following a green investment conference convened by Prime Minister Boris Johnson before the COP26 climate summit. Alongside her son Prince Charles and grandson Prince William, she welcomed guests such as US climate envoy John Kerry without wearing masks. The queen, dressed in a teal skirt and jacket with pearls, was seen smiling and chatting with guests, including Johnson. Despite her upcoming 70th year on the throne, she is known for her strong health. The most recent time she was known to have stayed in the hospital was in 2013 due to symptoms of gastroenteritis. In 2018, she underwent a successful surgery for an eye cataract and in 2003, she had a knee operation. Although royal officials avoid discussing health issues, she has continued to fulfill her official duties despite the death of her husband, Prince Philip, in April at the age of 99. Her advancing age has led her to delegate more responsibilities to Charles and other members of the royal family. Recently, she was spotted using a walking stick in public for the first time since her knee operation. She not only suffered the loss of her husband, whom she referred to as her source of support, but also faced the resignation of her second son Prince Andrew due to his connection to US financier Jeffrey Epstein, a convicted sex offender who died in a Manhattan jail in 2019. Additionally, her grandson Prince Harry and his wife Meghan have distanced themselves from royal duties by relocating to Los Angeles and criticizing Buckingham Palace. Queen Elizabeth's upcoming significant event is welcoming world leaders to the opening of COP26 in Glasgow at the end of the month.