Apparent tolerance to methamphetamine’s sleep-related effects depended on both the use schedule and the outcome measured. In a 2017 experiment involving five rhesus monkeys, continuous daily self-administration produced a different pattern from schedules interrupted by two- or four-day breaks. Sleep fragmentation also behaved differently from sleep efficiency and latency under one interrupted schedule.
The study supports an outcome-specific account of adaptation. It does not establish that tolerance means normal sleep physiology, that breaks make methamphetamine use safe, or that a person showing fewer outward effects is unimpaired.
Which animals were studied?
Berro, Andersen and Howell studied three adult male and two adult female rhesus monkeys weighing 9–15 kg. All five had extensive methamphetamine experience, consistently self-administering the drug five to seven days per week over the preceding four years, apart from interruptions between experiments.
This was not an experiment in drug-naive animals. The five monkeys were studied repeatedly; days, activity readings and protocol blocks do not increase the independent subject count. The small sample also does not support a reliable male–female comparison.
Animals lived under a 12-hour light/dark cycle, with lights on at 7 a.m. and off at 7 p.m. Activity-based baseline measures were recorded for a week before a protocol, with a further baseline week between protocols after self-administration stopped.
What methamphetamine exposure was available?
The drug was racemic (+/−) methamphetamine hydrochloride, administered intravenously, with doses expressed as the hydrochloride salt. During a daily 60-minute morning session, completing 20 operant responses delivered 0.03 mg/kg in 0.5 mL over three seconds, followed by a 60-second timeout.
The 0.03 mg/kg figure is a dose per infusion, not the entire session dose. Individual average intake ranged from 0.36 to 1.38 mg/kg per session; the reported group mean was 0.81 ± 0.05 mg/kg per session, expressed as mean ± standard error.
Intake did not differ significantly across days within the protocols. The authors also found no significant correlation between intake and sleep disruption or daytime activity under the studied schedules. These null findings limit an explanation based simply on progressively consuming less drug.
Which schedules were compared?
| Schedule | Pattern | Number of drug-access days |
|---|---|---|
| Continuous | Daily sessions for 14 consecutive days | 14 |
| Five days on, two days off | Three weekly blocks over 21 days | 15 |
| Three days on, four days off | Three weekly blocks over 21 days | 9 |
Calendar days and drug-access days are different denominators. The interrupted schedules also differ in both break length and the number of consecutive sessions, so a simple comparison should preserve the complete regimen rather than attribute everything to one isolated feature.
Were sleep stages measured?
No. Collar-mounted Actiwatch sensors recorded movement. Software generated activity-based sleep estimates: efficiency was the percentage of the dark phase classified as sleep; latency was the interval from lights-off to the first sleep bout; fragmentation indexed brief immobile bouts.
These were explicitly sleep-like behavioral measures. They were not electroencephalographic sleep staging, polysomnography or direct measurements of restorative sleep. Quiet wakefulness and movement-related classification limitations matter when interpreting an apparent return toward baseline.
The five animals also differed substantially at baseline: reported sleep-efficiency estimates ranged from approximately 47% to 84%. Normalizing results to each animal’s baseline helps compare change, but does not establish that every baseline represented the same sleep quality.
What changed during continuous daily access?
For the 14-day protocol, the first five days showed significantly reduced sleep efficiency, increased latency and increased fragmentation relative to baseline. Those measures no longer differed significantly from baseline when the final five days were examined together. Average daytime activity similarly rose in the early block but not significantly in the late block.
The authors interpreted the pattern as development of tolerance with repeated intake exceeding five consecutive days. A nonsignificant late-block comparison, however, does not independently prove physiological equivalence to baseline or complete absence of a drug effect.
Did the two-day-break schedule show the same pattern for every endpoint?
No. In the five-days-on, two-days-off protocol, efficiency and latency differed significantly from baseline in the first five-day block but not the last. Sleep fragmentation remained significantly elevated in the analyzed blocks. Broadly saying that all sleep effects disappeared would erase that finding.
Under the three-days-on, four-days-off schedule, significant disruption of efficiency, latency and fragmentation was present across the three drug-access blocks. The authors concluded that interruptions attenuated development of tolerance, with the longer breaks maintaining more of the measured effects.
These are patterns in a small repeated-measures experiment. They do not establish an individualized human schedule or a precise number of days after which every effect will recur.
Does adaptation establish reduced impairment or risk?
No driving task, collision endpoint or human functional-impairment assessment was performed. Daytime movement counts do not measure judgment, divided attention or safe performance. Tolerance on an actigraphy-derived measure also cannot establish tolerance to cardiovascular, psychiatric or other effects that were not the study’s endpoints.
The study’s pharmacological interpretation is further limited by species differences and the animals’ long exposure histories. The reported doses are not human blood concentrations, and the experiment provides no forensic concentration cutoff.
The useful conclusion is narrower: schedule and endpoint changed the apparent course of adaptation despite relatively stable intake. Preserving the fragmentation finding, the activity-based measurement and the five-animal denominator prevents “tolerance” from becoming an unsupported claim that all drug effects or risks disappeared.
Scientific source
Berro LF, Andersen ML, Howell LL. Assessment of tolerance to the effects of methamphetamine on daytime and nighttime activity evaluated with actigraphy in rhesus monkeys. Psychopharmacology. 2017;234(15):2277–2287. doi:10.1007/s00213-017-4654-1. Full-text author manuscript. Historical original nonhuman-primate experiment.
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