It is well documented that an adult’s core body temperature varies throughout the day in what is referred to as ‘diurnal temperature variation’. So rather than a stable 37° C / 97° F (say) the temperature fluctuates about 1° C / 2° F a day with lowest levels are around 4 a.m. and the highest in the late afternoon, between 4:00 and 6:00 p.m. (assuming the person sleeps at night and stays awake during the day). This is shown in the chart below.
Unfortunately, little research has been done on children’s body temperature variations with respect to adults but with regard to infants, from about 4 weeks of age, circadian rhythms begin to emerge, as evidenced by a dip in core temperature in the early morning. By age 3 months, the infant’s brain begins the circadian release of melatonin. By age 6 months, the period, amplitude, and phase activity of infants are believed to be about the same as those of adults, although infants exhibit a different sleep pattern (i.e., ‘‘multimodal’’) from that of adults. By approximately 1.5 years of age, toddlers give up their morning nap, and by age 6 years, children experience only nocturnal sleep. And again, unfortunately, little research concerning the effects of napping on the circadian rhythm of temperature in children has been undertaken.
Though it is clear from several studies that the occurrence of febrile seizures follows the daily temperature variation pattern of adults with the results of a Japanese study titled “Diurnal Variation in Febrile Convulsions” looking specifically at the time of day of the febrile seizures for 326 children aged between 6 months and 6 years. Shown below are the results of that study for the time of day of the seizure.
Suffice to say that a child’s body temperature is usually between 36.5°C / 97.7° F and 37.5°C / 99.5° F depending on the time of day. And if it is already elevated in the morning, while potentially still below the often cited 37.5° C / 99.5° F “fever” definition, the probability of a febrile seizure in the afternoon is increased.
Similarly research has shown that the time of year affects the probability of febrile seizures, most likely due to seasonal variations (e.g. flu cycles etc), however more research is required to understand these seasonal affects. Shown below are the seasonal results from a US study that looked at 100 consecutive admissions for febrile seizures to a university affiliated tertiary care hospital. The maximum frequency was in the spring and winter months with the lowest frequency in summer months.