Abstract Chronobiology and chronopharmacology have documented for six decades that biological processes and drug responses are periodic, with characteristic timescales spanning minutes to months. These periods are currently classified empirically — ultradian, circadian, infradian — without a unifying algebraic theory predicting which periods are admissible and why. We propose that the admissible biological periods are precisely the divisors of 120, the order of the timing group Z₁₂₀ = Z₃ × Z₅ × Z₈ derived from the T144 framework (Bénard 2026a–e). We demonstrate that 7 out of 9 well-established biological cell-turnover periods (erythrocyte 120 days, platelet 10 days, neutrophil 6 days, T-lymphocyte 30 days, intestinal epithelium 5 days, keratinocyte 40 days, circadian 24 hours) are exactly divisors of 120. The two exceptions — the menstrual cycle (~29.5 days, approximately 30 = 120/4) and the annual photoperiodic cycle (365 days, not a divisor of 120) — have algebraic explanations within the framework. The 7-day clinical week, which dominates pharmacological protocol design, corresponds to Z₇, a prime that is coprime to 120 and constitutes the spectral ghost of the Clifford tower. We conclude that chronobiology is not governed by arbitrary Zₙ subgroups but specifically by the subgroup lattice of Z₁₂₀, and that this constraint has immediate, testable implications for pharmacological timing. Keywords: chronopharmacology, Z₁₂₀, biological periodicity, subgroup lattice, T144, circadian, ultradian, infradian, Zn algebra, spectral ghost, pharmacological timing