If you work the night shift, taking your temperature can be tricky, but not impossible. Try to abide by the same rules of basal body temperature measurement as after a full night’s sleep. Measure your temperature after at least three hours of sleep, just after waking up, before you start any activity. If you go to bed in the morning and sleep until the afternoon, take your temperature as soon as you wake up, after the longest period of sleep.
As with the morning measurement, it's important to take your temperature every day at the same time. If this is not possible, then take your temperature after at least three hours of sleep, and note down the time, so that you know that the measurement may be erroneous. More often than not, despite the changes, your chart will be biphasic and ovulation will be marked correctly.