Introduction
The implementation of shift work systems is not uncommon in many workplaces.  Twenty-four hour operations are put in place to maximize usefulness of expensive technologies and to increase profitability.  Under normal conditions, the heart rate, body temperature, blood pressure, adrenaline  production, physical capabilities and mental alertness increase by day and decrease by night.  Shift work, night work in particular, forces the worker to invert the “activity-rest” cycle.

A contentious issue in employment is the prohibition of women workers in night work.  This limitation is not to be aimed specifically at protecting the reproductive capacity of women.  However, revisions in international standards have increased the opportunity for women to work under a shift system.  Recent trends in research have focused on identifying factors predictive of tolerance to shift work.  Assumptions have been put forward that rapid adjustment of circadian functions is related to shift work tolerance.

Objectives
This study compared the health and well being of females working in day and night shift in a yarn manufacturing company.  Specifically, female workers in day ad night work were compared with respect to the following: a) vitals signs, to include heart rate, heart rate variability, and body temperature; b) health complaints; and c) sleeping pattern, alertness at work, perception of soundness of sleep and other sleep-related parameters.

The psychosocial attributes and characteristics of the subjects were also described.  The data gathered may serve as basis for future research and to focus attention on the possible effects of shift work on the health of women workers.

Methodology
Fifteen female workers from a year manufacturing company participated in the study.  Questionnaires were administered and physiologic parameters were monitored for the 15 subjects from the yarn manufacturing company.  The subjects were initially monitored for 3 consecutive days from Day 1 to Day 3 of the night work schedule.  Follow up of the subjects was done after 30 days when they assumed work during the day shift schedule.  Similarly, the subjects were monitored for 3 consecutive days from Day 1 to Day 3 of the day shift schedule.

Results
The age of the study subjects ranged from 19 to 59 with a mean 37 years.  Most of the female employees who participated in the study have reached high school level of education.  The most common response for going on shifts (Mean: 3.3) was that shift work is an essential part of the job.  The subjects also answered that compensation is higher when working shifts (Mean: 2.5).  Notwithstanding, all of the subjects preferred daytime job to shift work.

The results of the present study demonstrated the pattern expected for the physiologic measures during day work.  The body temperature and heart rate exhibited the evening trough and morning peak patterns.  The sleep duration of workers during the day shift was noted to be stable for the 3 consecutive days of the survey.  During the night shift, the results revealed that the subjects initially had body temperature patterns that are similar to the normal diurnal rhythm.  The measured temperature tended to become lower as the night shift progressed.

However, an apparent adjustment started fro Day 3 when body temperatures started to rise, peaking in the latter hours of the shift.  The same holds true for the heart rate when the pattern started to flatten on the 2^nd day of night work.  The present study also assessed the autonomic influences on heart rate by the frequency domain method of measuring heart rate variability.  The results of the present study showed no significant differences between day and night wok low frequency to high frequency ratios in the 4 subjects who were monitored using the holter ECG machines.

Conclusion and Recommendations
Marginal changes in the heart rate and temperature demonstrated by the study may already suggest tolerance to shift work.  However, it is believed that physical activity, stress, sleep deprivation may produce some form of shift in the patterns of these variables.  The long-term changes in the physiologic parameters may be investigated to determine the extent of phase shift or inversion that will be attained.

Methodological adjustments in future studies should consider protracted 24-hour monitoring of heart rate variability to ascertain the diurnal pattern and pattern shifts.  The study design may also be modified to account for the healthy shift worker survivor bias where unhealthy workers may be forced to give up jobs that involve shifting schedule leaving behind workers who are healthy.

 The results of the present study may suggest tolerance of the female subjects to night work.  This can be viewed from the marginal changes in the body temperature and heart rate.  The results, however, also demonstrated increasing severity of night work-related symptoms with continued night work.  Since “perfect” adaptation is not common, it may be rational to seek the best strategy in work organization and occupational health service planning considering scientific evidences.