Body Temperature Minimum (BTM) in Relation to Phase Response Curve to Light
The body temperature minimum (BTM) serves as a critical reference point in the phase response curve to light, functioning as the inflection point that determines whether light exposure will cause a phase delay or phase advance of the circadian rhythm. 1
Definition and Timing of BTM
- BTM refers to the nadir (lowest point) of the core body temperature rhythm in the 24-hour circadian cycle, typically occurring approximately 1-2 hours before a person's habitual wake time 2
- BTM is considered a reliable marker of circadian timing and is often used as a reference point when designing light therapy interventions 1
- In healthy young adults with typical sleep schedules, the BTM usually falls in the latter part of the sleep period 2
BTM's Role in the Phase Response Curve
- The phase response curve (PRC) to light describes how the timing of light exposure relative to internal circadian phase determines the direction and magnitude of circadian shifts 1
- Light administered before the BTM typically delays circadian phase (shifts it later) 1
- Light administered after the BTM typically advances circadian phase (shifts it earlier) 1
- The BTM serves as the critical "inflection point" or crossover point between delaying and advancing effects of light on the circadian system 3, 2
Clinical Significance of BTM
- Understanding the timing of an individual's BTM is essential for properly timing light therapy interventions for circadian rhythm disorders 1
- For treating delayed sleep phase disorder, light should be administered after the BTM to advance the circadian rhythm 1
- For treating advanced sleep phase disorder, light should be administered before the BTM to delay the circadian rhythm 1
- The magnitude of phase shifts depends on light intensity, duration, spectral composition, and timing relative to the BTM 1
Measuring and Estimating BTM
- Direct measurement of core body temperature requires continuous temperature monitoring, typically using rectal temperature sensors 4
- Skin temperature rhythms can also be used as a less invasive alternative to estimate circadian phase 4
- In clinical settings, the BTM is often estimated based on sleep timing, with the assumption that it occurs 1-2 hours before habitual wake time 2
- Individual differences in chronotype (morning vs. evening preference) affect the timing of the BTM relative to clock time 5
Practical Applications
- Light therapy timing should be personalized based on an individual's estimated BTM to maximize effectiveness 5
- For night shift workers, understanding the relationship between light exposure and BTM is crucial for facilitating circadian adaptation 3, 5
- Maximum phase shifts of approximately 2 hours can be achieved with a single 3-hour bright light exposure properly timed relative to the BTM 6
- Larger phase shifts (4-7 hours) can be achieved with consecutive days of appropriately timed light exposure 6
Factors Affecting BTM
- Sleep-wake patterns directly influence the timing of the BTM 2
- Light exposure history can alter the timing of the BTM 1, 5
- Individual differences in chronotype affect the timing of the BTM, with evening types typically having later BTMs 5
- Age-related changes can affect the timing and amplitude of the core body temperature rhythm 2
Understanding the relationship between BTM and the phase response curve to light is essential for effectively treating circadian rhythm disorders and optimizing light therapy interventions to improve sleep, alertness, and overall health.