Submarine Environments:
Life Without Sunlight and What It Does to the Body
US submarines operate on an 18-hour day with no natural light, a dual zeitgeber loss the circadian system cannot entrain to. What the research shows and what submariners can do.
Key Takeaways
- US submarines operate on an 18-hour day, a cycle the SCN cannot entrain to, since it adjusts at only 0.5–1.0 hours per day and requires a 24-hour-period zeitgeber.
- Complete absence of sunlight removes the primary circadian entraining signal. Without blue-spectrum lighting, submariners lose both natural and artificial zeitgebers simultaneously.
- Marando et al. 2023 scoping review (13 studies): total sleep time was preserved at 5.46–7.89h[1], but circadian misalignment was greater on non-24-hour schedules and cognitive performance degraded.
- Chabal 2024 US Navy operational study: 24-hour circadian-aligned watchbill improved sleep efficiency, reduced fragmentation, and was rated more favorably, proving schedule reform is feasible.
What Makes Submarines Different
The dual zeitgeber loss
The human circadian system normally entrains to light: photons hit the retina, travel via the retinohypothalamic tract to the SCN, and reset the biological clock each morning. This light signal is so powerful that it can override the internal free-running period and keep the 24-hour clock synchronized to the solar day even when sleep timing is irregular.
Submarines remove this mechanism entirely. At depth, there is no sunlight. The artificial lighting environment of a submarine is defined by fluorescent and LED fixtures historically designed for functional visibility, not circadian signaling. The melanopsin-containing retinal ganglion cells (ipRGCs) that carry the circadian signal to the SCN respond primarily to blue-spectrum light (~480nm); submarine lighting that lacks this component provides minimal entrainment signal even if bright enough to see by.
The consequence: submariners lose both their natural entrainment signal (sunlight) and, in most historical configurations, any effective artificial substitute. Their circadian clock free-runs on its internal period, approximately 24.1–24.2 hours for most people, gradually drifting out of phase with the ship’s schedule.
Free-running without sunlight
The free-running period
In complete isolation from light-dark cues, the human circadian clock free-runs at its intrinsic period, typically 24.1–24.2 hours. In a submarine on an 18-hour day, even a clock running at exactly 24 hours completes 4 full cycles while the ship completes 3.2. There is no stable phase relationship possible.
The 18-hour day arithmetic
The traditional US Navy 6-on/12-off schedule creates a non-24-hour zeitgeber conflict: the schedule signals a cycle that the SCN physically cannot follow. The result is chronic circadian misalignment, not a temporary transition like jet lag, but a sustained mismatch that does not resolve over the deployment.
Why artificial light doesn’t help (usually)
Submarine lighting historically designed for visibility lacks the blue-spectrum component (peak ~480nm) that drives SCN entrainment via the melanopsin pathway. Bright but blue-poor light provides minimal circadian signal even when it’s bright enough to read by.
Who this applies to most
- Active duty submariners: Every patrol involves circadian disruption that accumulates over weeks without adequate environmental countermeasures.
- Veterans with submarine service: Post-submarine sleep disorders are frequently unrecognized as occupationally caused. Circadian assessment should be considered for veterans with non-restorative sleep who served on submarines.
- Submarine medical officers and corpsmen: The only personnel positioned to advocate for schedule and lighting reforms from within the chain of command.
- Coast Guard offshore patrol crew: Similar isolated environments with inadequate natural light exposure and non-aligned watch schedules.
The 18-hour day
The traditional US Navy submarine watch schedule is based on an 18-hour cycle: 6 hours on watch, 12 hours off. In practice, net sleep time is often 6–7 hours, preserved in total duration, but occurring at varying clock times as the schedule cycles through the 24-hour clock. The 18-hour day creates a non-24-hour zeitgeber conflict the SCN cannot follow.
| Schedule | Cycle length | 24-hr alignment | Circadian verdict |
|---|---|---|---|
| 6/12 (18h day) | 18 hours | Cannot entrain | Persistent misalignment |
| 6/12 (24h aligned) | 24 hours | Aligned | Better performance, preferred |
| 4/8/8 (20h day) | 20 hours | Cannot entrain | Misaligned, less severe than 18h |
| 4/8 (12h aligned) | 24 hours | Aligned | Optimal for circadian |
What the Research Shows
The Marando et al. (2023) scoping review synthesized 13 studies[1] of submarine watchkeeping schedules.[1] Total sleep time was preserved across most schedule types (5.46–7.89h), but circadian misalignment was significantly greater on non-24-hour schedules, and longer off-watch periods were consistently associated with better cognitive performance.
Chabal et al. (2024[2], Applied Ergonomics) documented the first US Navy operational evaluation of a 24-hour circadian-aligned watchstanding schedule in submarines.[2] Sailors on the aligned schedule achieved comparable total sleep duration but significantly better[2] sleep efficiency, reduced fragmentation, and self-reported improvements in alertness and wellbeing, confirming operational feasibility.
Erez et al. (2025[3], Journal of Sleep Research) studied Israeli Navy submariners on a 20-hour rotating schedule and found increased premature responses on cognitive testing, heightened tension during early-morning watches, and elevated risk-taking propensity during afternoon watches.[3]
Light as a circadian countermeasure
Guyett et al. (2024[4], Sleep) conducted a randomized trial of circadian-informed lighting on Royal Australian Navy submarines.[4] The lighting intervention, providing appropriate blue-spectrum stimulation during the biological morning phase and reduced blue-spectrum exposure pre-sleep, accelerated circadian adjustment to the night-work schedule by several days. This is the first controlled trial of circadian lighting in an operational submarine environment.
The leverage point: submarines are the one environment where light conditions can be completely controlled. A submarine can, in principle, achieve better circadian outcomes than any operational environment with partial natural light intrusion.
What the critics say
The total-sleep-time preservation finding has been used to argue that schedule reform is unnecessary. The research does not support this interpretation. Cognitive performance degradation persists even when total sleep hours appear adequate, because circadian misalignment degrades sleep quality and timing independently of duration. The critique applies to how we measure adequacy; it does not undermine the evidence for schedule-related performance impairment.
What the Evidence Doesn’t Say
Long-term health consequences. The chronic disease outcomes of career-long submarine service: cardiovascular risk, metabolic disruption, cancer risk from chronic circadian misalignment. Have not been directly studied in submarine veteran populations.
Optimal lighting protocol. Guyett et al. (2024) provides proof of concept, but the optimal spectrum, timing, and intensity parameters for submarine circadian lighting have not been definitively established for fleet implementation.
Women submariners. Women have served on US submarines only since 2011. The circadian and sleep research is almost entirely pre-integration and predominantly male. Whether outcomes differ for women submariners is not yet established.
Clinical Implications
| Application | Evidence | Strength | Notes |
|---|---|---|---|
| Evaluate submarine veterans for circadian rhythm disorders, not just standard insomnia | Non-24-hour schedule plus light deprivation produces distinct circadian pathology that may not respond to standard CBT-I | Moderate (Marando 2023 scoping review) | Consider actigraphy and DLMO assessment in treatment-refractory cases |
| Document submarine service duration in VA sleep evaluations | Submarine patrols are a quantifiable circadian exposure that supports service connection for sleep disorders | Clinical practice | Ask specifically about number of patrols and approximate schedule used |
| Advocate for circadian lighting on submarine fleets | Submarines are uniquely amenable to light-based circadian interventions, complete light control is available | Moderate–strong (Guyett et al. 2024 RCT) | Clinical personnel on submarines are the most effective advocates for this change |
| Screen for shift work disorder criteria in post-submarine veterans | Non-24-hour schedule produces clinical shift work sleep disorder, a recognized diagnostic entity | Moderate | Apply ICSD-3 criteria for shift work disorder when evaluating this population |
What Can You Do?
| How to Implement | Expected Benefit (and Why) | Evidence Strength | Context Notes |
|---|---|---|---|
| Get a sleep evaluation that includes circadian assessment | |||
| Ask your VA provider for actigraphy monitoring (2 weeks of wrist-worn activity tracking) and, if available, DLMO measurement | Identifies circadian phase delay or free-running rhythm as the primary driver of sleep complaints, because standard sleep history cannot detect circadian disorders that require timing data | Moderate (clinical practice) | Actigraphy is widely available in VA sleep clinics; mention submarine service history when requesting it |
| Use morning bright light to re-anchor the circadian clock post-patrol | |||
| Immediately after returning from a patrol, use outdoor morning sunlight or a 10,000 lux light therapy lamp for 30 minutes at your target wake time | Provides the light-based phase-advancing signal the circadian clock lost during the patrol, because the SCN requires strong morning light to synchronize to the 24-hour day | Moderate–strong (chronobiology principle) | Combine with a fixed wake time; light therapy is most effective when consistent |
| Report the circadian nature of sleep complaints explicitly | |||
| Tell your provider: “I served on submarines. My schedule was an 18-hour day with no sunlight. I believe I have a circadian rhythm disorder, not just insomnia.” | Directs the provider toward appropriate diagnostic tests and treatment, because standard CBT-I sleep restriction may worsen circadian disorders | Clinical practice | This framing opens the door to circadian assessment and appropriately timed melatonin treatment |
How to Use AI With This Information
When to Work With a Professional
Seek VA sleep medicine evaluation if:
- Sleep problems have persisted for more than 4 weeks after completing a patrol or separating from submarine service
- You notice a consistently drifting sleep pattern, falling asleep progressively later or earlier over time, which may indicate a free-running rhythm disorder
- Standard insomnia treatments (sleep hygiene, medications) have been tried without benefit
- You experience extreme difficulty maintaining wakefulness during the day despite adequate sleep duration
FAQ’s
Can the submarine schedule permanently damage the circadian system?
Prolonged circadian disruption can produce lasting changes in circadian clock gene expression and melatonin pathway function, but full recovery is generally possible with adequate environmental entrainment. The recovery timeline depends on cumulative disruption, longer careers with more patrols may require longer re-entrainment periods.
Are there circadian countermeasures available to submariners now?
The evidence supports two interventions with current feasibility: schedule alignment (moving from 18-hour to 24-hour schedules, which several Navy programs are implementing) and circadian lighting (managing spectrum and timing of artificial lighting). Both have been validated in operational submarine environments and neither requires pharmaceutical intervention.
Does melatonin help with submarine-related sleep problems?
Melatonin can help re-anchor the circadian clock after returning from a patrol, taken at the target destination bedtime. For active patrols, its use is more complex, the correct timing depends on the current phase of the misaligned clock. Coordinate with submarine medical personnel.
REFERENCES
- Marando I et al. (2023). The sleep, circadian, and cognitive performance consequences of watchkeeping schedules in submariners: A scoping review. Sleep Med Rev, 72, 101845. doi:10.1016/j.smrv.2023.101845
- Chabal SA et al. (2024). Life onboard a submarine: Sleep, fatigue, and lifestyle behaviors on a circadian-aligned watchstanding schedule. Appl Ergon, 119, 104321. doi:10.1016/j.apergo.2024.104321
- Erez D et al. (2025). The impact of a 20-h rotating watch schedule on cognitive and mood states in submarine operations. J Sleep Res, 34(4), e14400. doi:10.1111/jsr.14400
- Guyett A et al. (2024). A circadian-informed lighting intervention accelerates circadian adjustment to a night work schedule in a submarine lighting environment. Sleep, zsae146. doi:10.1093/sleep/zsae146
- Skornyakov E et al. (2017). Sleep and performance in simulated Navy watch schedules. Accid Anal Prev, 99(B), 422–427. doi:10.1016/j.aap.2015.11.021
- Guo J et al. (2020). Circadian misalignment on submarines and other non-24-h environments. Military Medical Research, 7(1), 39. doi:10.1186/s40779-020-00268-2
- Paul MA & Love RJ. (2022). Comparison of Royal Canadian Navy watchstanding schedules. Mil Med, 187(3-4), e418–e425. doi:10.1093/milmed/usab047
- Shattuck NL & Matsangas P. (2016). Operational assessment of the 5-h on/10-h off watchstanding schedule. Ergonomics, 59(5), 657–664. doi:10.1080/00140139.2015.1073794
- Matsangas P & Shattuck NL. (2022). Self-reported sleep and sleep deficiency in US Navy warship sailors. J Sleep Res, 31(3), e13534. doi:10.1111/jsr.13534
- Fernandes C et al. (2025). Sleep in military submariners: Exploring dynamics in relation to a submarine mission. Behav Sleep Med, 23(4), 526–539. doi:10.1080/15402002.2025.2493650