There may be several factors, from fever to alcohol to lack of sleep, that could affect your waking temperature. Yet this doesn’t compromise your ability to rely on them while charting, because you ultimately want to identify a pattern of low and high temperatures, rather than focusing on individual ones. Outlying temperatures can be effectively dealt with by using the Rule of Thumb, which usually allows you to ignore them in interpreting your chart. In addition, you will always be able to rely on your other two signs to cross-check your fertility in situations such as these.