| CBFV (cerebral blood flow velocity) is lower in the morning than in the afternoon and evening. Two hypotheses have been proposed to explain the time of day changes in CBFV. First, CBFV changes are an evoked effect secondary to sleep associated processes, and second, time of day change in CBFV is due to an endogenous circadian rhythmicity driving CBFV lower at night independent of sleep. There is also a greater incidence of strokes, in the morning hours between 6 a.m. and 12 p.m. The aim of the study was to examine CBFV over 30 hours of sustained wakefulness to determine whether CBFV exhibits fluctuations associated with time of day. Nine subjects underwent a modified constant routine protocol. CBFV from the middle cerebral artery was monitored by chronic recording of Transcranial Doppler (TCD) ultrasonography. Other dependent variables included core body temperature (CBT), end-tidal CO2, blood pressure, and heart rate. Salivary dim light melatonin onset (DLMO) served as a measure of endogenous circadian phase position. A non-linear multiple regression, cosine fit analysis revealed that both the CBT and CBFV rhythm fit a 24 hour rhythm (R 2 = .79 and R2 = .50, respectively). FFT analysis revealed that all subjects showed a cycle of ∼24 hours in CBT and in seven out of nine subjects for CBFV. Salivary DLMO occurred 6--10 hours before CBT nadir in seven out of nine subjects. Aligning both rhythms revealed a 90 degree difference, though not significant (t = -0.52, p = 0.62), between the CBT and CBFV rhythm. Once aligned, the rhythm of CBFV closely tracked the rhythm of CBT as demonstrated by the substantial correlation between these two measures (r = .76, p = <.01). In conclusion, time of day variations in CBFV have an approximately 24 hour rhythm under constant conditions, suggesting regulation by a circadian oscillator. The 90 degree-phase angle difference between the CBT and CBFV rhythms may help explain alertness in the evening before habitual bedtime and may contribute to sleep inertia in the morning after waking up. |
