  Background and Objectives: Many biological processes are modulated by rhythms
on circadian and multidien timescales. In focal epilepsy, various seizure
features, such as spread and duration, can change from one seizure to the next
within the same patient. However, the specific timescales of this variability,
as well as the specific seizure characteristics that change over time, are
unclear.
  Methods: Here, in a cross-sectional observational study, we analysed
within-patient seizure variability in 10 patients with chronic intracranial EEG
recordings (185-767 days of recording time, 57-452 analysed seizures/patient).
We characterised the seizure evolutions as sequences of a finite number of
patient-specific functional seizure network states (SNSs). We then compared SNS
occurrence and duration to (1) time since implantation and (2) patient-specific
circadian and multidien cycles in interictal spike rate.
  Results: In most patients, the occurrence or duration of at least one SNS was
associated with the time since implantation. Some patients had one or more SNSs
that were associated with phases of circadian and/or multidien spike rate
cycles. A given SNS's occurrence and duration were usually not associated with
the same timescale.
  Discussion: Our results suggest that different time-varying factors modulate
within-patient seizure evolutions over multiple timescales, with separate
processes modulating a SNS's occurrence and duration. These findings imply that
the development of time-adaptive treatments in epilepsy must account for
several separate properties of epileptic seizures, and similar principles
likely apply to other neurological conditions.
