5D Library › The Default Mode Network, the Entropic Brain and Neural Oscillations · 中文
Chapter 5 The Pineal Gland: From Melatonin to Calcite
A philosopher’s gland
The pineal gland is a structure about the size of a grain of rice, near the centre of the brain. René Descartes, looking for a single organ where soul and body could meet, chose it because, unlike most brain structures, it is not paired, and he described it in his Passions of the Soul of 1649 as the principal seat of the soul’s interaction with the body (Descartes 1649). That was a philosophical proposal from an era before cells, neurons or hormones were understood. It is worth separating clearly from what is now known about the gland, and from more recent speculation.
Melatonin and the night
Evidence
Melatonin was isolated from the bovine pineal gland in 1958 (Lerner et al. 1958). Julius Axelrod and Herbert Weissbach identified the enzyme that completes its synthesis, by methylating N-acetylserotonin (Axelrod and Weissbach 1960); Axelrod shared the 1970 Nobel Prize in Physiology or Medicine for work on neurotransmitters. The pathway runs from the amino acid tryptophan to 5-hydroxytryptophan, then to serotonin, then (by the enzyme AANAT) to N-acetylserotonin and finally to melatonin (Claustrat et al. 2005).
Melatonin is made mainly at night. Light falling on the retina, detected chiefly by a special class of light-sensitive ganglion cells, signals to the suprachiasmatic nucleus, the body’s master clock, and through a chain of connections that runs down to the upper spinal cord and back up via the superior cervical ganglion, it switches off the night-time activation of AANAT in the pineal. Melatonin is cleared from the blood with a half-life of under an hour (Claustrat et al. 2005), so its level follows its production closely.
Evidence
In controlled laboratory studies, the human melatonin system is sensitive to light at levels far below daylight: dim room light at night already produces a large share of the maximal suppression that very bright light produces (Zeitzer et al. 2000).
Calculation
A minute-by-minute model of night-time melatonin from 18:00 to 08:00. Synthesis is switched on at 21:00 and off at 07:00, clearance has a half-life of 45 minutes, and light at the eye between 21:00 and 23:00 reduces synthesis according to a saturating curve that is half-maximal at 100 lux. The suppression curve is illustrative, chosen to be consistent with the laboratory finding above.
import math
HALF_LIFE_MIN = 45 # plasma melatonin half-life, roughly 30–60 min
K_CLEAR = math.log(2) / HALF_LIFE_MIN
def suppression(lux, l50=100.0, n=1.5):
"""Share of night-time synthesis switched off by light at the eye.
Illustrative curve: half-maximal near ordinary room light (see text)."""
return lux ** n / (l50 ** n + lux ** n)
def night(evening_lux, light_from=21.0, light_to=23.0):
"""Minute-by-minute model from 18:00 to 08:00. Synthesis is 'on' from
21:00 to 07:00 (unit rate per minute) and reduced while the light is on.
Returns total melatonin (area under the curve) and the 23:00 level."""
m, area, at23 = 0.0, 0.0, 0.0
for i in range(14 * 60):
h = 18 + i / 60 # hours after midnight; 31 = 07:00 next day
rate = 1.0 if 21 <= h < 31 else 0.0
if light_from <= h < light_to:
rate *= 1 - suppression(evening_lux)
m += rate - K_CLEAR * m
area += m
if abs(h - 23) < 1e-9:
at23 = m
return area, at23
dark_area, dark23 = night(0)
print("evening light 21:00–23:00 level at 23:00 night total")
for lux in (0, 10, 100, 300, 1000):
a, l23 = night(lux)
print(f"{lux:6d} lux {100 * l23 / dark23:8.0f} % {100 * a / dark_area:9.0f} %")
# evening light 21:00–23:00 level at 23:00 night total
# 0 lux 100 % 100 %
# 10 lux 97 % 99 %
# 100 lux 51 % 90 %
# 300 lux 18 % 82 %
# 1000 lux 5 % 80 %
The level at 23:00 falls by about half with ordinary room light and almost completely with bright light, but the total over the night falls much less, because melatonin recovers quickly once the light is off. A model like this explains why evening light matters for the timing of melatonin; it does not by itself say anything about sleep quality, which depends on the circadian clock and much else.
Calcite in the pineal
Evidence
The pineal gland often contains mineral deposits, the “brain sand” (corpora arenacea), which are mainly calcium phosphate in the form of hydroxyapatite. In 2002, Baconnier and colleagues described a different kind of deposit in human pineal glands: microcrystals of calcite, calcium carbonate, typically less than 20 µm long, with a complex twinned structure; they noted that, apart from the otoconia of the inner ear, this was the only known non-pathological calcite in the human body (Baconnier et al. 2002). In a companion conference paper they proposed that the crystals might be responsible for second-harmonic generation of light observed in pineal tissue, and that their twinning “may lower their symmetry” enough to allow piezoelectricity (Baconnier and Lang 2002).
Here the physics needs care. Piezoelectricity, the generation of electric polarisation by mechanical stress, is described by a third-rank tensor: . In a crystal that has a centre of symmetry, every index changes sign under inversion, so the tensor must equal its own negative, and is therefore zero. Calcite’s crystal class is centrosymmetric. Bulk calcite is therefore not piezoelectric. The possibility raised by Baconnier and colleagues depends on the twinned microstructure breaking that symmetry, which they presented as a possibility, not a measurement. Later measurements on natural calcite found only a weak, residual response, attributed to the centrosymmetric crystals themselves (Guerin et al. 2018).
Calculation
The symmetry argument in one line, and the acoustic resonance of a calcite micro-crystal, , with the speed of sound from a typical stiffness and density.
import math
# 1. Symmetry. Calcite (CaCO3) belongs to a centrosymmetric crystal class.
# Piezoelectricity links a vector (polarisation) to a second-rank tensor
# (stress): P_i = d_ijk σ_jk. Under inversion every index flips sign, so a
# centrosymmetric crystal must satisfy d = −d, i.e. d = 0.
d = 1.0
print("inversion test: d_ijk → (−1)^3 d_ijk =", (-1) ** 3 * d, "; only d = 0 is unchanged")
# 2. Acoustic resonance of a micro-crystal: f ≈ v / (2 L), v = √(c / ρ).
c11, rho = 1.45e11, 2710 # stiffness (Pa) and density (kg/m³), textbook order
v = math.sqrt(c11 / rho)
print(f"longitudinal sound speed ≈ {v / 1000:.1f} km/s")
for L_um in (2, 10, 20):
f = v / (2 * L_um * 1e-6)
print(f" {L_um:2d} µm crystal: fundamental ≈ {f / 1e6:5.0f} MHz "
f"= {f / 40:.1e} × a 40 Hz gamma rhythm")
# inversion test: d_ijk → (−1)^3 d_ijk = -1.0 ; only d = 0 is unchanged
# longitudinal sound speed ≈ 7.3 km/s
# 2 µm crystal: fundamental ≈ 1829 MHz = 4.6e+07 × a 40 Hz gamma rhythm
# 10 µm crystal: fundamental ≈ 366 MHz = 9.1e+06 × a 40 Hz gamma rhythm
# 20 µm crystal: fundamental ≈ 183 MHz = 4.6e+06 × a 40 Hz gamma rhythmSo a crystal a few to twenty micrometres long has its fundamental mechanical resonance in the hundreds of megahertz, millions of times higher than any brain rhythm. Even if the crystals were piezoelectric, they could not resonate with the brain’s oscillations in the way that is sometimes claimed in popular writing.
Hypothesis · not established
Not established. Calcite microcrystals in the human pineal gland have no physiological function and are a by-product of mineral deposition, like hydroxyapatite brain sand. The hypothesis predicts that the number or size of calcite microcrystals is unrelated to measures of pineal function, such as night-time melatonin output, once age is taken into account. It would be refuted if, in a study measuring both in the same people, calcite content predicted melatonin output independently of age and of hydroxyapatite calcification.
Note
The phrase “activating the pineal gland” appears in many popular sources, sometimes linked to “third eye” ideas. Nothing in the evidence reviewed here supports claims that the pineal gland can be activated, decalcified or tuned by any practice, sound or product, and this book makes no such claim.
References
Fan, Y. (2026). The Default Mode Network, the Entropic Brain and Neural Oscillations: 5-HT2A Receptor Kinetics, Pineal Calcite Microcrystals and the Computational Neuroscience of Meditative Attention [默认模式网络、熵脑与神经振荡] (Version 1.0.0). 5D Library. https://doi.org/10.5281/zenodo.23697320 · Licensed under Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0).