increased pollution from rapid industrialization and increases in the smoking rate are 
 gradually focusing public attention on respiratory disorders .
breathing , the major function 
 of the lung , is the process that alternately performs inspiration and expiration with gas 
 exchange that is essential for humans life1 .
deteriorating lung function is a major cause of death among south 
 koreans , and the number of patients with poor lung function is increasing2 .
managing lung function can improve dyspnea 
 and enhance quality of life3 , consequently 
 public attention to respiratory physiotherapy is increasing . in respiratory physiotherapy , 

the evaluation of 
 pulmonary function is performed to assess the lung s mechanical functions , volume , and 
 capacity4 . specifically , peak expiratory 
 flow ( pef ) , forced vital capacity ( fvc ) , forced expiratory volume in 1 second 
 ( fev1 ) , fev1/fvc , and forced expiratory flow between 25 and 75% of 
 vital capacity ( fef2575% ) are related to the degree of disability and short - term 
 and long - term prognoses in a variety of respiratory diseases and are frequently used as 
 assessment tools5 .
in addition , maximal 
 inspiratory pressure ( mip ) and maximal expiratory pressure ( mep ) are performed to evaluate 
 pulmonary strength .
these are known to be useful clinical indicators of the natural progress 
 of chronic obstructive pulmonary disease ( copd ) patients6 . various human organs , including the lung , exhibit circadian rhythms in which physiological 
 functions are controlled according to a specific cycle .
recently , postural control has been 
 reported to be related to diurnal variation9 , and diurnal variations exist in respiration10 .
the diurnal variation in respiration is known as the 
  morning dip  phenomenon , and evaluation of breathing at 4:00 pm gives the highest values , 
 while the lowest values are measured in the morning11 .
a review of previous studies of the diurnal variations of 
 pulmonary evaluation suggests conflicting viewpoints .
hetzel12 reported changes in pulmonary function and pulmonary strength with 
 time , whereas aguilar et al.13 reported 
 that pulmonary function and pulmonary strength showed no statistically significant changes 
 with time .
the purpose of this study was to identify the changes in pulmonary function and 
 pulmonary strength associated with time of day .
the subjects were 20 healthy adults ( 11 men , 9 women ) who had no cardiopulmonary - related 
 diseases .
the mean age , mean height , and mean weight of the subjects were 23.553.09  years , 
 169.909.61  cm and 64.4013.48  kg , respectively .
the subjects were explained the purpose of 
 this study and they voluntarily signed informed consent forms before participation in this 
 study .
this study obtained the approval of the bioethics committee of catholic university of 
 pusan ( cupirb-2014 - 010 ) .
the subjects pulmonary function and pulmonary strength were evaluated at three times , 9:00 
 am , 1:00 pm , and 5:00 pm , based on the hospital s working environment .
the tests were 
 performed at least 1 hour after the subjects had eaten a meal .
pulmonary function was 
 evaluated using microlab ( micro medical ltd . , uk ) .
each subject was seated and looked 
 straight ahead with the mouthpiece of the measurement device inserted in the mouth and a 
 nose clip fixed on the nose .
pulmonary strength was evaluated using microrpm ( micro medical ltd . , uk ) to measure mip and 
 mep .
the mip and mep were measured while the subjects were seated and looked straight ahead 
 with the mouthpiece of the measurement device inserted in the mouth .
inhalation and 
 exhalation were repeated three times , and the highest value was selected . if differences of 
 more than 10% were found among the measured values , these values were excluded .
the data collected during the process were encoded and analyzed using spss for windows ver . 

in addition , repeated measures 
 analysis of variance ( avona ) was conducted to compare the differences in pulmonary function 
 and pulmonary strength and contrast tests was used to compare between times of day .
the results of pulmonary function at the different times of day are shown in table 1table 1.the changes of pulmonary function with time of day9:00 am1:00 pm5:00 pmfvc3.450.123.550.133.600.12fev1 * 3.150.103.350.103.450.09fef25754.400.224.500.194.450.18unit=. * : statistically significant ( p<0.05 ) .
fvc and fef2575% showed no statistically 
 significant differences among the different times of day .
the results of pulmonary strength 
 at the different times of day are shown in table 
 2table 2.the changes of pulmonary strength with time of day9:00 am1:00 pm5:00 pmmip72.04.373.43.976.54.4mep74.35.774.44.676.94.6unit = cmh2o .
different superscripts in a row indicate a significant 
 difference .. mip and mep showed no statistically significant differences among the 
 different times of day , but comparative testing of each variable found statistically 
 significant differences between measurements taken at 9:00 am and 5:00 pm .
human diurnal variations are controlled by the 
 suprachiasmatic nucleus located in the anterior hypothalamus , which serves the role of the 
 main circadian pacemaker that controls almost all human organs and behaviors15 , 16 .
bagg and hughes11 reported that diurnal 
 variations exist in the peak expiratory flow ( pef ) : the highest value was observed at 4:00 
 pm and the lowest value was observed in the morning , the morning dip phenomenon .
their results displayed the morning dip phenomenon with 
 significantly reduced values being observed in the morning . in the present study , the 
 morning dip phenomenon could be observed .
they did not tabulate the numerical 
 values of their results , thereby limiting comparisons with our study .
medarov et al.18 found that when fvc and fev1 
 were values measured in the afternoon they showed were the highest values .
the difference in 
 fvc between the highest and the lowest values was 11.9% , and the difference in 
 fev1 between the highest and the lowest values was 15.7% . in the present study , 
 fvc and fev1 measured in the morning were about 4.2% and 8.7% less than their 
 respective values measured in the afternoon .
the fef2575% measured in the 
 morning was about 1.1% less than that measured in the afternoon , also displaying the morning 
 dip phenomenon .
teramoto et al.17 
 reported statistically significant diurnal variations in mip and mep , but , as mentioned 
 above , they did not tabulate the numerical values of their results , thereby limiting 
 comparisons with our study .
. the mip and mep 
 values in the morning were a little higher than those measured in the afternoon .
while their 
 study reported statistically significant differences , the small subject number limits its 
 generalizability .
aguilar et al.13 
 measured the diurnal variations of mip and mep of healthy adults for 12 hours .
mip decreased 
 about 4.6% , mep increased around 1.3% during the day , and mep exhibited the morning dip 
 phenomenon .
in the present study , mip and mep measured in the morning were respectively 
 about 5.9% and 3.3% less than the mip and mep measured in the afternoon .
thus , they 
 demonstrated the morning dip phenomenon , but the differences were not statistically 
 significant .
the results of the present study demonstrate that pulmonary function and pulmonary strength 
 show changes with time of day , confirming the morning dip phenomenon , though the differences 
 were small .
although the differences were small , the morning dip phenomenon appeared 
 throughout the experiment , sometimes showing statistically significant difference . 

therefore , we consider the time of measurement is a matter of clinical concern . this study 
 was performed with healthy asian adults in their 20s as subjects ; therefore , the results 
 can not be generalized to the elderly or patients with lung disorders .
some studies have 
 reported that pulmonary evaluation measurements are influenced by gender , ethnicity , age , 
 and height , and thus these factors should be taken into account in testing5 , 20 . 

follow - up studies should be conducted considering other factors , such as the age and disease 
 of patients .
 [ purpose ] the purpose of this study was to identify changes in pulmonary function and 
 pulmonary strength according to time of day . 
 [ subjects and methods ] the subjects were 20 
 healthy adults who had no cardiopulmonary - related diseases . 
 pulmonary function and 
 pulmonary strength tests were performed on the same subjects at 9:00 am , 1:00 pm , and 5:00 
 pm . 
 the pulmonary function tests included forced vital capacity ( fvc ) , forced expiratory 
 volume in 1 second ( fev1 ) , and forced expiratory flow between 25 and 75% of 
 vital capacity ( fef2575% ) . 
 pulmonary strength tests assessed maximal 
 inspiratory pressure ( mip ) and maximal expiratory pressure ( mep ) . 
 [ results ] fev1 showed 
 statistically significant differences according to time of day . 
 other pulmonary function 
 and pulmonary strength tests revealed no statistical differences in diurnal variations . 
 
 [ conclusion ] our findings indicate that pulmonary function and pulmonary strength tests 
 should be assessed considering the time of day and the morning dip phenomenon . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION