At least 140,000 patients were put at risk last year owing to NHS incidents in the middle of the night, figures showed.
Trips, falls and medication errors between 10pm and 6am meant thousands had their safety compromised when they were supposed to be asleep.
The study, for Reader's Digest, is based on a breakdown of statistics from the National Patient Safety Agency (NPSA).
They relate to the period April 2006 to March 2007 and show that 22% (more than 140,000) of incidents linked to patient safety - where the time of day was known - happened during the night.
During this time there are no scheduled operations, consultations or tests.
Accidents such as slips, trips and falls were the most frequently reported night-time incident, accounting for 60% of the total.
Problems with admission, transfer or discharge represented 6% of night-time incidents while treatment errors accounted for another 6%. A further 6% of all incidents during the night were to do with medication errors.
It is already known that more than 700,000 "patient safety incidents" occurred in the NHS during the year 2006/07.
In total, 6,558 incidents resulted in severe harm and another 40,665 caused moderate harm to patients. There were a total of 2,929 deaths due to patient safety incidents.
The time of day during which the incidents occurred was known for 81% of the 784,476 total reported to the NPSA for 2006/07. Incidents occurred at night in 22% of these known cases (around 140,000) and during the day in 78% of cases (around 500,000).
