Module 3 · Entrainment

Light Entrainment

The human circadian period is ~24.2 h on average — close to but not exactly 24 hours. Every day, light synchronises (entrains) the clock to the external 24-hour cycle. This module covers how light is detected by the retina for clock purposes — through a distinct system from image-forming vision — and how phase response curves quantify the effects of timed light exposure.

1. The Retinohypothalamic Tract (RHT)

Light information reaches the SCN through a direct projection from a small population of retinal ganglion cells — the intrinsically photosensitive retinal ganglion cells (ipRGCs). Discovered by Berson (2002) and Hattar (2002), these cells express their own photopigment, melanopsin (OPN4), and respond directly to light without requiring rod/cone input. They constitute ~1% of retinal ganglion cells.

Melanopsin’s spectral peak is ~480 nm — in the blue range. This is why blue light (short wavelength) is especially potent for phase-shifting the clock and suppressing melatonin, while red/long-wavelength light has little effect. The retinoid-based, cytosolic-Gq-coupled melanopsin signalling is slow (hundreds of milliseconds) compared with rod/cone phototransduction, which makes ipRGCs function as long-timescale light integrators.

2. Light in the SCN: The Molecular Pathway

ipRGC axons release glutamate and PACAP at SCN neurones. The cascade:

Glutamate binds NMDA/AMPA receptors, Ca²⁺ enters.
Ca²⁺ activates kinases (CaMKII, PKA), which phosphorylate CREB.
Phospho-CREB binds CRE elements in Per1 and Per2 promoters, rapidly inducing their transcription.
The sudden rise in PER proteins shifts the current oscillator phase: an advance if light comes at late night, a delay if light comes at early night.

This is the molecular basis of phase-shift by light: injecting PER expression at the wrong time in the TTFL cycle re-sets the oscillator.

3. The Phase Response Curve

Light’s effect depends on when it arrives. The phase response curve (PRC) plots the phase shift produced by a light pulse as a function of the pulse’s circadian time:

Early night (~CT14–18): light causes phase delays (up to ~2 h).
Late night / early morning (~CT22–4): light causes phase advances (up to ~1.5 h).
Subjective day (~CT8–14): “dead zone”, minimal effect. The clock is insensitive during the daylight hours to which it is already entrained.

Human PRC was mapped by Khalsa et al. (2003) using carefully timed 6.7-hour bright-light exposures. The curve is biphasic (“weak type 1” PRC in Pittendrigh’s classification). Clinically, the PRC guides phototherapy protocols for DSPD, SAD, and jet lag.

Simulation: Human Phase Response Curve

Python

script.py44 lines

import numpy as np, matplotlib
matplotlib.use('Agg')
import matplotlib.pyplot as plt

# Phase Response Curve for light in humans (schematic, based on Khalsa et al. 2003)
# Light delivered in the evening / early night causes phase delays (negative)
# Light in the late night / early morning causes phase advances (positive)
# Near solar noon: "dead zone" (negligible effect)

t = np.linspace(0, 24, 500)
# Approximate human PRC shape: negative sine-like centered around CT18, positive around CT6
CT = (t + 12) % 24   # circadian time offset
# Maximum phase delay ~2h at CT 16-20, max advance ~1.5h at CT 22-4
delay = -2.0 * np.exp(-((t - 20)/3)**2)      # hours of delay, centered 8 pm
advance = 1.5 * np.exp(-((t - 4)/3)**2)      # hours of advance, centered 4 am
# Wrap-around
advance2 = 1.5 * np.exp(-((t + 24 - 4)/3)**2) + 1.5 * np.exp(-((t - 28)/3)**2)
response = delay + advance + advance2

fig, ax = plt.subplots(figsize=(12, 6), facecolor='#0a0a1a')
ax.set_facecolor('#111827'); ax.tick_params(colors='#cbd5e1')
for s in ax.spines.values(): s.set_color('#334155')
ax.grid(True, color='#334155', alpha=0.3)

ax.fill_between(t, response, 0, where=(response>0), color='#34d399', alpha=0.3, label='Phase advance (waking earlier)')
ax.fill_between(t, response, 0, where=(response<0), color='#f87171', alpha=0.3, label='Phase delay (going to bed later)')
ax.plot(t, response, color='#fbbf24', lw=2.6)
ax.axhline(0, color='#94a3b8', lw=0.8)
ax.axvline(12, color='#94a3b8', ls=':', alpha=0.4, label='Solar noon')
ax.axvspan(22, 24, color='#1e1b4b', alpha=0.15)
ax.axvspan(0, 6, color='#1e1b4b', alpha=0.15, label='Subjective night')

ax.set_xlabel('Time of light exposure (h local time)', color='#cbd5e1')
ax.set_ylabel('Phase shift (hours)', color='#cbd5e1')
ax.set_title('Human light phase-response curve (Khalsa et al. 2003, schematic)',
             color='#c4b5fd', fontweight='bold')
ax.legend(facecolor='#1e293b', edgecolor='#334155', labelcolor='#cbd5e1', loc='lower right')
ax.set_xlim(0, 24)
ax.set_xticks([0, 6, 12, 18, 24])

plt.tight_layout()
plt.savefig('output.png', dpi=120, bbox_inches='tight', facecolor='#0a0a1a')
print('Phase Response Curve (PRC): evening light delays, morning light advances')
print('Dead zone at mid-day (solar noon): clock insensitive to light')

Click Run to execute the Python code

Code will be executed with Python 3 on the server

4. Non-Image-Forming Vision

ipRGCs project not only to the SCN but also to the olivary pretectal nucleus (pupillary light reflex), the intergeniculate leaflet, the ventrolateral preoptic nucleus (sleep regulation), and the amygdala (mood). This is non-image-forming vision: retinal processing that does not create a visual percept but regulates alertness, mood, and physiological state. It explains why individuals with outer retinal blindness can still have pupillary responses to light and can still be circadian-entrained. It also explains why bright morning light has rapid mood-elevating effects: direct ipRGC input to limbic structures.

5. Non-Photic Zeitgebers

Light is not the only Zeitgeber. Scheduled feeding can entrain peripheral clocks independently of light (Module 5). Social cues, exercise, and temperature cycles contribute weaker but measurable phase information. In experimentally blinded humans, scheduled melatonin ingestion at 22:00 can maintain entrainment. Olfactory cues entrain rodents in DD. These non-photic channels gain importance in the blind, in shift workers, and in polar populations during prolonged daylight or darkness.

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