5 min readMay 26, 2026 01:48 PM IST First published on: May 26, 2026 at 01:48 PM ISTHeat waves occur in India from March to June or July, and warnings are typically based only on the maximum surface air temperature (daytime). Daytime-only heatwaves (DHW) are increasingly turning into nighttime-only (NHW) and day-night compound heatwaves (CHW), and the conventional “hot-day and cool-night” no longer holds.The dangers of warm nights and high indoor temperatures are thus increasingly gaining recognition. A recent analysis reveals that warm nights in northeast, northwest and peninsular India increased by two to eight days/decade and are projected to increase by 10–13 days/decade from 2015 to 2100. The forecast for the far-future (2080–2100) scenario: Warm nights (and days) are expected to rise sevenfold.AdvertisementAlso Read | 10630296High night temperatures have a bearing on both sleep quantity and quality, and the nocturnal recovery of the body from diurnal heat stress. Body temperature falls after sleep onset, and the set point for thermoregulation is adjusted to a lower level. Humid heat exposure (during sleep time) suppresses the sleep-evoked core body temperature, raises the thermal load, and increases wakefulness.Accounting for two thermal indices — hot night excess and hot night duration — a recent study has established strong evidence of an increased mortality risk associated with hot-night indices in most regions across the world. Significantly, this was found to be independent of any association with daytime temperature. A study in Japan observed a higher mortality risk from hot nights in early summer compared to late summer. The “hot night excess” has thus been associated with increased hospital admission and mortality risks across the world from conditions such as stroke, cardiovascular conditions and dementia.Even a night’s lack of sleep has negative implications for cognitive ability, immune system, mood changes, quality of life and social activity. The National Time Use Survey (TUS) conducted by India’s Ministry of Statistics and Programme Implementation in 2024 reveals that the hours spent on essential sleep (ages 15-59) vary from 8.29 hours for rural men to 7.76 hours for employed urban women. Significantly, Indian women in their 30s, across both rural and urban segments, sleep 29 minutes less than men in that age group.AdvertisementIndoor overheating is especially critical in bedrooms and is a key determinant of restorative sleep. The intersection of urban heat exposure and housing vulnerability is gaining increasing attention. Recent observations in Chennai revealed that indoor temperatures remained in the 32°-35.5°C range, and even exceeded 35 °C. A study in Salta, Argentina, found that nearly two-thirds of bedrooms recorded strong or extreme overheating during heatwaves. Prevailing relative humidity adds another layer of heat stress to this. Media reports indicate that the India Meteorological Department (IMD) is considering revising the criteria for declaring heatwave conditions in coastal regions.Several factors commonly contribute to high indoor temperatures. External shades have been a traditional consideration for residential housing sites, but are increasingly less so. Awnings, solar screens and blinds can help with indoor cooling. Cross-ventilation helps in cooling, but modern apartment buildings are often deficient in this count; opening windows during high heat diurnal hours is not advisable. Rooms in low-income settlements typically have low ceilings with not enough space for “stratification” (warmer air near the ceiling and cooler air closer to the floor). Indoor electrical gadgets such as televisions, lights, computers or cooking equipment radiate heat and switching to low-energy/thermally efficient equipment reduces indoor heat to some extent.Permanent indoor alterations are costly, difficult to make and not suitable for the vast majority who are tenants. Measures to improve shading include installing moveable/removable shading above, around, or on windows and doors and shades for open spaces such as verandas. Installing blinds and placing leafy green indoor plants by windows improve internal shading in rooms that are exposed to direct sunlight. Ceiling fans certainly improve ventilation, but tend to spread the heat around if the temperature is above 35°C.you may likeIt is generally difficult and expensive to cool the entire home; instead, one or two rooms are best kept cool, and as little time as possible is spent on rooms that receive direct sunlight or kitchens. A shower before bed reduces body temperature slowly and avoids the risk of overheating. A glass of cold water before bed and loose clothing also help.While heat-related mortality risk and advisories focus on peak daytime temperatures or exposure across the entire day, it is difficult to adapt to the consequences of nighttime exposure and high indoor heat. India needs to respond to this challenge, taking into account both climatic and socioeconomic contexts.The writer is chairperson, Centre of Social Medicine and Community Health, JNU, and a collaborator in the Wellcome Trust-supported ‘Economic and Health Impact Assessment of Heat Adaptation Action: Case studies from India’. Views are personal