Patients are staying half a day longer in hospital now than they did in 2008.
The average stay last year was 6.4 days, up from 5.9 days in 2008, according to Ministry of Health (MOH) figures.
Professor Chee Yam Cheng, head of the National Healthcare Group, said reasons for this include hospitals taking in only more serious cases - largely because of the current bed crunch - and an increasing number of day surgery cases which are soaking up short-stay patients.
With a growing population, there has also been an increase in the number of patients needing hospitalisation. Between 2008 and last year, the number of people admitted to public hospitals went up by 15 per cent - from almost 258,000 to nearly 297,000 at the six acute public hospitals.
The longer average stay means hospital beds are being used for about 150,000 days a year more than they were in 2008 - an 8 per cent increase.
Prof Chee said the criteria for admitting patients were more lax in the past, given the greater availability of beds, while today, public hospitals are "more strict".
More operations and treatments can now be carried out as day procedures, with no hospital stay needed, such as the delivery of intravenous antibiotics.
Prof Chee said: "In the past, they were admitted while the intravenous antibiotics were administered maybe three or four times per 24 hours over five or 10 days.
"The newer antibiotics need only one daily dose so the patient can come as an outpatient instead of staying in hospital."
Dengue treatment has also improved as doctors become more familiar with the disease. Despite dengue cases reaching almost 12,000 so far this year, such patients occupied less than 1 per cent of beds.
Meanwhile, hospitals are seeing more and more elderly patients, who take longer to recover, Prof Chee said.
Their care is complicated because they often have more than one medical problem and "close monitoring is essential as... (their condition) can quickly turn bad".
MOH said patients aged 65 and older stayed about 8.2 days at a time last year, compared with the average of five days among younger patients.
With Singaporeans living longer, the trend towards longer hospital stays is likely to continue.