One in three people who work night shifts complains of bad health, including gastrointestinal disorders, illnesses associated with high blood pressure, or sleeplessness, according to a government survey.

The Health, Labor and Welfare Ministry said the finding comes from surveys last year on the health of night-shift employees.

The study, which polled 8,000 employees and 9,000 companies, found that 21 percent of responding employees work night shifts. Of those, 36 percent said they had health problems after they began working nights, including 17 percent who either suffered gastrointestinal disorders, high blood pressure-related ailments or sleep disorders.

Twenty-two percent of the 9,000 companies that responded said they have night shifts. By industry, truck drivers and other transport industry employees formed the bulk of late-night workers, accounting for 43 percent of the total, followed by the manufacturing industry, at 18 percent.

The survey found that the rate of workers suffering maladies increased with the length of time spent on night shifts -- 24 percent in the case of employees who have worked nights less than six months, rising to 40 percent in the case of those who have worked nights more than three years.

The labor ministry said 67 percent of the companies with night shifts said they have health policies to address work-related conditions, and 77 percent said they offer employees free medical consultations.

Only 16 percent of the companies with night shifts said they allow employees to change shifts or reduce the frequency of night shifts in response to health-related complaints.

In total, 12,000 employees and 11,000 companies were solicited for responses.