Peter Nelson
Director of Sports Science, Boston Bruins
Sleep is the most powerful “recovery tool” we already own—and also the one that is collectively misunderstood the most. Here are five sleep truths that cut through the usual advice and give you a few high-value practical takeaways you can actually put to use tonight.
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1) Sleep is Not a Credit Card: Stop Trying to “Pay Off Your Debt”
Most people treat sleep like money: underspend all week, then “catch up” on the weekend. The problem with this approach is that your physiology doesn’t run on that type of accounting system. In one 2019 controlled lab study by Depner and colleagues at UC-Boulder, participants were asked to adhere to sleep-wake cycles that mimicked this type of “real life” pattern (short sleep on weekdays and sleeping in on the weekend); not only did the “weekend recovery” fail to reverse markers of metabolic damage, but in some measures it actually worsened it.
The practical takeaway is simple: consistency beats compensation. Treat sleep more like medicine than money: if you miss a dose, don’t double it later—get back on schedule at the next scheduled dose. Protect a consistent wake time, keep your sleep window steady, and avoid huge weekend swings (“social jet lag”), which are increasingly linked to worse cardiometabolic outcomes, even when total sleep time looks “okay.”
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2) Optimal Sleep Timing: A Marriage of Wall Clock and Body Clock
There is a popular saying that “an hour of sleep before midnight is worth two after midnight”. The veracity of this statement, however, is a bit nuanced: it is not actually factual in a literal sense, but it’s not totally fiction either:
“Fact”: Your body doesn’t really care what the clock says, but it does care about light and dark, which have an obvious association with clock time. Earlier bedtimes usually correlate with less artificial light exposure late at night and more morning light exposure, which strengthens circadian “day/night signaling” across hormones, temperature, appetite/glucose handling, and cardiovascular function. In a large UK Biobank study, sleep onset between 10:00–10:59 PM was associated with the lowest incidence of cardiovascular disease, while sleep onset after midnight was associated with higher risk, even after adjusting for total sleep duration, irregularity, and other risk factors.
“Fiction”: The beneficial effects of earlier sleep times are not universal, however, as significant inter-individual differences have been shown to exist. These differences are primarily driven by chronotypes, which is the term given to legitimate biological differences in internal clock and cyclical biological rhythms that can shift an individual more towards a “morning person” or “night owl” disposition—controlled lab work shows “evening types” run about 2–3 hours later than morning types on sleep timing and circadian markers like melatonin/temperature rhythms. So an evening type can have high-quality sleep on a later schedule if it’s consistent and they’re not forced into early wake-ups. It should be noted that chronotype is not a free pass for extreme lateness: in that same UK Biobank study, later sleep timing related to poorer mental health outcomes in a way that led the authors to conclude that people should start sleeping before 1 AM, despite chronotype preference.
Bottom line: What really matters is whether you’re sleeping during your own “biological night” (your circadian “dark phase”). Don’t worship midnight—anchor your sleep to your day and your cycles. Keep a stable wake time, get bright outdoor light early, keep evenings dim, and bias bedtime earlier if you can do it without sacrificing total sleep.
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3) Melatonin Isn’t Magic: It’s a Specific Solution, Not a Sledgehammer
Melatonin is one of the most widely recognized sleep aids, yet remains one of the most misunderstood. Your brain naturally produces this hormone as a signal that it’s “biological night,” and appropriately administered exogenous melatonin can help when the main problem is circadian timing—think jet lag or shift-work-related sleep timing issues.
But for the classic “I can’t sleep because I’m stressed/wired/waking up a lot” insomnia profile, melatonin is often the wrong tool for the job. The American Academy of Sleep Medicine has guidelines suggesting clinicians not use melatonin to treat chronic insomnia in adults because the effect sizes in randomized control trials are small and inconsistent.
If you want to be “supplement-smart,” the guiding principle should be “match the solution to the problem.” Melatonin is mostly indicated for augmenting sleep timing/realignment of circadian rhythms; for “staying asleep” issues, other supplements should be considered. For example, several small trials suggest that the amino acid glycine can improve subjective sleep quality and next-day fatigue, and the amino acid theanine has demonstrated an ability to promote a relaxed state conducive to quality sleep. Regardless of the specific nature of any sleep-related issue(s) that you might try to address, the primary focus should be fundamentals first: you often get more mileage by targeting ambient temperature, alcohol and meal timing, stress/arousal level management, and screening for sleep-disordered breathing.
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4) Jet Lag Starts Before Takeoff: Treat It Like Prehab, Not Rehab
Most people treat jet lag as a post-arrival problem, opting to attempt to realign their light exposure once they get to their destination, or, even worse, suffer through the deleterious effects until they self-resolve. These individuals would be better served by adopting a different strategy: pre-gaming their sleep-wake adjustments, so that they become proactive instead of reactive.
The CDC’s 2026 Yellow Book explicitly breaks jet lag management into phases and notes that shifting sleep toward the destination time zone in the days prior to departure can reduce adjustment time—for example, shifting sleep and wake times about 1 hour per day for 2–3 days pre-trip (earlier for eastward travel, later for westward) can help soften the internal biological adjustments that your body needs to make when you arrive.
Key actionable moves that actually matter:
- Pre-flight: If possible, start nudging bedtime/wake time toward destination time for a few days.
- During flight: Set your watch to destination time and time sleep, meals, and light exposure accordingly; stay hydrated; avoid excess alcohol; be cautious with long half-life sedatives.
- After arrival: Use timed light exposure as your main circadian lever (seek/avoid light strategically), keep naps short if needed (20–30 minutes), and prioritize daylight exposure at the destination.
An important addendum: direction matters. Most people adapt faster to westward travel (delaying the clock) than eastward (advancing it), which is one reason eastbound trips often feel harsher.
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5) Pulse Check: Your Watch Says HRV, Your Heart May Disagree
HRV (heart rate variability) has become a popular metric featured in commercial sleep tracking systems, and with good reason—there’s a large body of evidence linking HRV to a host of different physiological functions and health outcomes. It is traditionally derived from electrocardiogram (ECG) R‑R intervals—electrical events in the heart. Many consumer wearables, especially wrist devices, instead use photoplethysmography (PPG) optical sensors to detect pulse waves and thus compute pulse rate variability (PRV). PRV can correlate with HRV in ideal, resting conditions, but it is not the same signal and can diverge due to vascular factors, signal artifacts, and the fact that it’s measuring blood volume changes—not cardiac electrical activity.
In a large 2024 clinical population study using a device capable of both ECG and PPG, PPG‑derived PRV showed poor agreement with ECG‑derived HRV metrics and tended to underestimate common HRV indices—supporting the conclusion that PRV is an invalid “drop-in replacement” for HRV in many contexts.
Having said that, that doesn’t mean that your sleep tracker is useless—you just need to understand the limitations of the data that it provides. Best practices to make the most of your wearable’s outputs:
- Use “HRV” as a within-device trend, not an absolute truth.
- Compare apples-to-apples: same device, same conditions (overnight/rest), same routine.
- Don’t let a single number override your basics (sleep time, consistency, training load, stress).
If you want “cleaner” HRV, you generally need ECG-based measurement (e.g., a chest strap) and consistent measurement conditions.
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If you take away just one concept from this list, let it be this: protect a consistent sleep schedule, then use light (morning brightness, evening dimness) to reinforce it. Sleep doesn’t need to be perfect—but it does need to be predictable.
And if you’re routinely waking unrefreshed, snoring loudly, or struggling with insomnia for months, that’s not a “willpower” problem—it’s something that’s worth talking about with a qualified clinician or sleep specialist.
