At around 10pm, residents near the pharmaceutical company base in Southdown View Way, Worthing, said they could hear alarms going off and tannoy announcements.
This morning, a GSK spokesman confirmed what the noise was about. They said: "The safety of our employees is our utmost priority so we have a fire safety plan in place which requires the site to run an annual day time and night time fire evacuation drill.
"At 10pm on Thursday 4th October one of these safety drills took place at GSK’s Worthing site, which was completed successfully.
"The West Sussex Fire & Rescue Service and neighbours in the immediate vicinity were notified ahead of the drill taking place.”