Infant sleep: 4–12 months

Around 4 months, your baby's sleep architecture changes permanently. Before this, infants spent ~50% of sleep in active REM and transitioned between stages without fully waking. After 4 months, sleep cycles become full adult-style cycles (~50-60 minutes at this age, growing to 90 minutes by school age) with brief micro-arousals between every cycle^[1].

This is the source of "the 4-month regression." The baby who slept 8-hour stretches at 3 months suddenly wakes every 45 minutes at 4 months because they've now hit a fully conscious micro-arousal between every cycle and no longer know how to re-settle. The fix is teaching independent sleep skills, not waiting it out^[3].

Independent sleep skills means falling asleep without active soothing — without nursing to sleep, rocking to sleep, or being patted down. The baby learns to navigate the awake-but-tired state and slip into sleep on their own. Once they have this skill, they re-settle through micro-arousals automatically and sleep stretches consolidate.

Wake windows at this age stretch from about 90 minutes (4 months) to 3 hours (12 months). Naps consolidate from 4-5 short naps to 2-3 longer naps. The morning nap is usually the most predictable; the afternoon nap is usually the longest; the third (cat) nap is the first to drop, typically around 8-9 months.

By 6 months, most infants are on a 2-3 nap day with 11-12 hours of nighttime sleep. By 9 months, 2 naps. The 2-to-1 nap transition starts as early as 12 months and as late as 18 months — that's a toddler-stage transition, not infant-stage.

Total daily sleep at this age is 12-15 hours from 4-6 months and 12-14 hours from 6-12 months^[2]. Within those bands, individual variation of ±1 hour is common. If your baby is happy, eating well, and developmentally on track, you're in the normal range.

Sleep training is teaching independent sleep skills. Multiple approaches work; pick what fits your family^[3]:

Cry-it-out (Ferber, extinction): place baby down awake, leave, return at increasing intervals. Effective in 3-7 nights for most families^[4]. Hardest emotionally for parents; minimal evidence of long-term harm in research^[3][4].

Chair method / camping out: sit by the crib, gradually move farther away over nights. Slower (2-3 weeks) but allows parental presence. The Gradisar 2016 RCT^[4] found camping out and cry-it-out equally effective at 12 months and equally safe at 5-year follow-up.

Pick up / put down: pick up to soothe, put back when calm. Repeats endlessly the first 2-3 nights. Works for some babies, exhausting for parents, and many sleep consultants now consider it less effective than the alternatives.

No-cry / gentle: gradual reduction of sleep associations (e.g., move from nursing-to-sleep to drowsy-but-awake over weeks). Slowest (4-8 weeks) and works best when started early.

The research is consistent that all of these work; family fit matters more than which method^[3]. Don't start before 4 months — the sleep architecture isn't ready. The 4-6 month window is the easiest time; later starts work but take longer.

The infant sleep environment is simpler than most marketing suggests. A safe firm surface (crib or bassinet, no inclined sleepers), the room dark to blackout level, white noise at safe volume (≤ 50 dB at 1 meter, machine ≥ 7 feet from crib^[5]), room temperature around 68-72°F (20-22°C), and a sleep sack instead of loose blankets.

Sleep sacks vs swaddles: swaddling stops when rolling starts, usually around 4 months. After that, sleep sacks (wearable blankets) replace swaddles. Don't use loose blankets in the crib until at least 12 months — the safe sleep guidance is firm on this^[2].

Things you don't need: pillows, stuffed animals, bumpers, weighted sleep sacks (recently flagged by AAP for safety concerns), incline positioners. The bare crib is the safest crib.

Short naps (under 45 minutes): often a wake-window mismatch. Try the upper end of the wake window for several days; if naps lengthen, you had it too short. Past 6 months, "30-minute intruder" wakings often happen at the cycle boundary — independent sleep skills resolve this.

Early waking (before 6 AM): usually means bedtime is too late or last nap is too late. Try moving bedtime 15 minutes earlier. If bedtime is already early, last nap might be ending too late.

Sleep regressions at 8-10 months: usually developmental leaps (crawling, pulling up, separation anxiety). Stay consistent with the routine; this passes in 2-3 weeks.

Bedtime resistance starting around 9 months: separation anxiety peaks here. A consistent wind-down routine, dim lights, and a transitional object (small comfort blanket, lovey introduced after 12 months for safety) help.