Winter sickness absence continues to peak earlier and more sharply than the much-publicised National Sickie Day, according to new UK workforce data tracking more than one million employees.
An analysis of 2025 absence records shows that the highest surge in sickness absence occurred on Monday 6 January, rather than the first Monday in February traditionally labelled as National Sickie Day. The findings suggest that winter illness, mental health pressures and post-holiday strain continue to shape absence patterns more than cultural folklore.
The data also raises questions for employers about how sickness absence is understood, monitored and supported during the most challenging months of the year.
January Mondays Drive Biggest Absence Spikes
The analysis was carried out by BrightHR, which examined absence data from more than 1.2 million employees across 120,000 UK businesses.
It found that sickness absence spiked by 182 percent on Monday 6 January, making it the single sickest day of the year. By contrast, National Sickie Day finished fourth for absences, marking the tenth consecutive year it has failed to top the annual rankings.
All of the ten highest absence days recorded in 2025 fell on Mondays, reinforcing the role of winter routines, fatigue and health pressures at the start of the working week.
Regional, Demographic Patterns Emerge
While National Sickie Day did not lead overall absence levels, some cities did see sharp localised increases on that date. Blackpool recorded the largest spike, followed by Norwich, Cambridge, Aberdeen and Leicester, each seeing absence increases of more than 200 percent compared with average daily levels.
The data also pointed to demographic trends. Gen Z employees were the most likely age group to take sickness absence during the year, with an average of four in five employees recording at least one period of sickness leave.
BrightHR said the findings show that absence patterns are not random and can reflect broader wellbeing pressures within specific regions or workforce groups.
Employers Urged to Look Beyond ‘Sickies’
Alan Price, chief executive officer at BrightHR, said the figures showed that National Sickie Day no longer reflected the reality of workplace absence.
“Although National Sickie Day itself has now officially popped its clogs, Mondays in winter are still officially the worst for absences, whether due to cold and flu viruses, seasonal pressures on mental health or employees getting creative with reasons why they need a day off,” he said.
Price said that understanding patterns was essential to supporting staff wellbeing rather than focusing on suspicion or blame. “Approaching wellbeing from an employee perspective means emphasising the importance of mental health to encourage a culture of openness and support,” he said.
Wellbeing Signals Hidden in Absence Data
Price added that sharp spikes in particular cities should prompt employers to ask broader questions about workforce wellbeing. He said absence data can act as an early warning signal when analysed properly.
“Alarm bells should be ringing in the cities where sickies spiked, a more than 200 percent increase is no coincidence. What are the signs that employers are missing?” he said.
He said tracking trends over time can help organisations identify emerging risks before they escalate into sustained productivity loss or employee burnout.
Financial Impact Set to Rise Under Day One Sick Pay
The data also highlights the cost implications of sickness absence for employers ahead of changes to Statutory Sick Pay. From April, sick pay will apply from the first day of absence, rather than after a waiting period.
BrightHR estimates that around 200,000 additional workers could be absent on the single sickest day next year, at a minimum cost of £5.13 million to UK employers for that day alone.
Price said employers would need to balance cost control with wellbeing support. “National Sickie Day is a drop in the ocean of extra considerations for employers,” he said, adding that absence trends should inform workforce planning rather than reactive management.
As winter pressures continue to affect physical and mental health, the data suggests that the real challenge for employers lies not in one symbolic day, but in sustained support throughout the colder months.