Sleep-dependent modulation of thalamic brain temperature estimates in humans
Mammalian thermal physiology is geared towards dissipating heat during sleep. Brain temperature typically decreases during non-rapid-eye-movement (NREM) sleep in all mammalian species measured to date, yet non-invasive confirmation in healthy humans is lacking: the only available human study used invasive recordings in epileptic patients without a wake control condition. Magnetic resonance…
Mammals naturally regulate their body temperature by releasing heat during sleep. Brain temperature is known to drop during non-rapid eye movement (NREM) sleep across various mammalian species. However, in humans, confirming this phenomenon non-invasively remains a challenge, with previous research relying on invasive measurements in epileptic patients without a control group observing wakefulness.
To address this issue, magnetic resonance spectroscopy (MRS) offers a non-invasive method for mapping brain temperature by measuring the temperature-sensitive chemical shift of water in relation to N-Acetyl-Aspartate as an internal reference.
The researchers conducted two primary studies: first, they validated the use of metabolite cycling (MC) in MRS, a technique that allows for simultaneous acquisition of metabolite and water information without traditional water suppression. This approach was successfully applied to determine brain temperature estimates in an ex vivo sheep head.
Second, they applied the MC sequence to analyze existing MRS data of the thalamus in nine young adults (n=9) who slept for up to 2 hours in the MRI scanner during the evening. The objective was to investigate any sleep-evoked changes in brain temperature estimates.
Among the nine participants, five also participated in a subsequent wake-only experiment, which enabled the researchers to control for the potential influences of circadian time and body posture on brain temperature estimates. Polysomnography was used to determine sleep and wake states, and it confirmed that participants successfully entered sleep during the scanner session.
The average duration of sleep consisted of 39.4±15.7 minutes in N2 sleep, 24.8±18.1 minutes in N3 sleep, 10.0±4.9 minutes in N1 sleep, and 37.0±19.6 minutes awake.
The researchers then examined the correlation between brain temperature estimates during sleep and while participants were awake within the same individual (R^2=0.29, P<0.05).
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